Method for displaying austenite grain size of cr-mo-v hot work die steel

Abstract

The application discloses a kind of Cr-Mo-V series hot die steel austenitic grain size display method, belongs to metal material metallographic inspection technical field.The present application is aimed at the problem that Cr-Mo-V series hot die steel grain is difficult to clearly and completely display in spheroidizing annealing state, provide a kind of Cr-Mo-V series hot die steel austenitic grain size display method, comprising: low-temperature austenitizing treatment;High-temperature tempering treatment;Polish, mechanical polishing, electrolytic etching and light polishing;Gradient cycle processing, and control electrolytic etching voltage, electrolytic etching time and light polishing time gradually decrease.The present application uses oxalic acid aqueous solution to carry out electrolytic etching, by the method of ladder type cyclic electrolytic etching+light polishing, strengthen the contrast difference between grain boundary and matrix, more clearly display austenitic grain boundary, obtain clear, contrast high grain boundary morphology, facilitate accurate grain size rating.

Description

Technical Field

[0001] This invention belongs to the field of metallographic inspection technology of metallic materials, specifically relating to a method for clearly and accurately displaying the austenite grain size of Cr-Mo-V series hot work die steel, especially in the spheroidized annealed state, through a combination of specific heat treatment and electrolytic corrosion. Background Technology

[0002] Cr-Mo-V series hot work die steels (such as H13 steel) are key materials for manufacturing dies for die casting, forging, and extrusion. Austenite grain size is one of the core indicators for evaluating the quality of this type of steel. Coarse grains will significantly reduce the material's toughness and resistance to thermal fatigue. Therefore, accurately and clearly displaying and rating the original austenite grain boundaries is crucial for material inspection, production process optimization, and die failure analysis.

[0003] Currently, the technology for etching austenitic grain boundaries in hot work die steels in the quenched and tempered state is relatively mature. One commonly used method is the hot etching method using supersaturated picric acid and corrosion inhibitors. However, when this method is applied to spheroidized annealed Cr-Mo-V hot work die steels, it faces the following problems: First, traditional methods cannot accurately reflect the original state. Hot work die steels are typically delivered in a spheroidized annealed state, with a microstructure consisting of a ferrite matrix and a large number of uniformly distributed granular carbides. If the traditional picric acid etching method is used, these undissolved carbides severely interfere with the etching process, resulting in blurred, discontinuous, or even completely unidentifiable austenite grain boundaries, making accurate grain size assessment difficult. To obtain clear grain boundaries, the annealed sample is usually re-austenitized and quenched at a higher temperature (e.g., 1000~1050℃), and then the resulting martensitic microstructure is etched. However, this secondary austenitization process carries the risk of grain growth, and what is actually displayed is austenite grains with a changed state, and the possibility of unclear grain boundaries still remains.

[0004] Secondly, picric acid is a high explosive, posing an extremely high risk of explosion when dry due to friction, impact, or heat. Laboratories typically require it to be prepared as an aqueous solution and stored and used while moist, necessitating extremely strict management and posing safety hazards. Furthermore, its acquisition and storage are difficult: due to its explosive properties, the procurement, transportation, and storage of picric acid are strictly controlled by public security departments, making it difficult for many ordinary enterprise laboratories to legally and conveniently obtain and store this reagent. Besides picric acid, most other methods utilize relatively complex chemical reagents.

[0005] CN119666677A discloses a method for detecting the austenite grain size of martensitic hot work die steel, comprising the following steps: (1) primary sample preparation; (2) sample heat treatment; (3) secondary sample preparation; (4) etching; and (5) taking metallographic photographs and performing grain size rating. This method comprehensively utilizes oxidation, etching, and metallographic methods, solving the problems of difficulty in detecting the austenite grain size of martensitic hot work die steel, easy grinding away of grain boundaries obtained by the single oxidation method, and unclear grain boundary manifestation in existing technologies, thus enabling more accurate and reliable effective rating of austenite grain size. However, the sample preparation process of this method is relatively complicated and requires secondary sample preparation. The sample is first polished and then heat-treated. After heat treatment, it needs to be polished again. During the secondary sample preparation, the test surface of the sample needs to be polished at an angle of 10-15° with the sandpaper plane, which is quite difficult to operate. More importantly, for some steel grades with a small amount of carbide precipitation, this method uses 4% nitric acid alcohol for corrosion, which cannot achieve preferential dissolution of grain boundary regions with irregular atomic arrangement and high energy, and cannot clearly and completely display the grain boundaries.

[0006] CN114086182A discloses a steel austenitic grain size etchant, its preparation method, and its application. The etchant includes a carboxyl compound, a surfactant, and a diluent. The carboxyl compound is either p-acetic acid or oxalic acid; the surfactant is either sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, or detergent; and the diluent is a mixture of water and ethanol. This method eliminates the need for high-temperature oxidation treatment and employs a direct hardening method to directly chemically etch grain boundaries, clearly revealing the grain boundaries of wheel steel, carbon structural steel, and low-to-medium carbon alloy steel samples. This solves the problems of environmental pollution and public safety concerns associated with picric acid reagents in existing technologies. However, this method uses relatively complex reagents, and the chemical etching method may result in incomplete grain boundary display. Chemical methods primarily reveal grain boundaries by corroding impurity element segregation and precipitated phases at the grain boundaries. When some grain boundaries lack these conditions, the grain boundaries may not be fully displayed.

[0007] CN118424815A discloses a method for characterizing the microcrystalline grain size of high thermal conductivity hot work die steel, comprising the following steps: quenching and tempering a sample of high thermal conductivity hot work die steel, grinding and polishing, then etching with an etchant, followed by observation of the original austenite grain boundaries and grain size rating under a metallographic microscope; wherein the composition and ratio of the etchant are: oxalic acid 3~9g, concentrated nitric acid 4~6ml, hydrochloric acid 4~6ml, acetic acid 2~5ml, anhydrous ethanol 100ml, and distilled water 10ml; the quenching treatment is performed at 1030℃~1080℃ for 30min~60min, followed by cooling; the tempering temperature is 560℃~620℃, and the tempering time is 1.5h~2.5h; the mass percentage composition of the high thermal conductivity hot work die steel is: C 0.05%~0.35%, Mo The composition is 2.6%~5.3%, Cr≤0.15%, Ni 0.01%~0.35%, with the remainder being iron and unavoidable impurities. This method uses an etchant to achieve grain boundary corrosion of high thermal conductivity hot work die steel at room temperature. However, the etchant contains multiple solutions such as oxalic acid, concentrated nitric acid, hydrochloric acid, and acetic acid, resulting in complex proportions. Furthermore, it involves quenching and tempering treatment on high thermal conductivity hot work die steel samples, and these quenching and tempering regimes are not suitable for spheroidized annealed Cr-Mo-V series hot work die steels. Additionally, it does not employ a multi-stage etching and polishing method, which is not conducive to a clearer and more complete display of grain boundaries.

[0008] Therefore, it is necessary to develop a method for displaying austenite grain size in Cr-Mo-V hot work die steels, especially those in the spheroidized annealed state. This method should be able to accurately preserve and display the original austenite grain boundaries; avoid the use of high-risk, difficult-to-obtain, or other complex reagents such as picric acid, thereby improving the safety and operability of the detection process; and obtain clear, high-contrast grain boundary morphologies for accurate grading. Summary of the Invention

[0009] To address the aforementioned technical problems, this invention proposes an easy-to-operate, clear, and accurate method for displaying austenite grain size in Cr-Mo-V hot work die steels (especially in the spheroidized annealed state). This method effectively highlights grain boundaries while preserving the original austenite grain state of the material to the greatest extent possible.

[0010] This invention provides a method for displaying the austenite grain size of Cr-Mo-V series hot work die steel, which includes the following steps: A. Austenitizing treatment: The Cr-Mo-V series hot work die steel test samples were subjected to heat treatment at 850~950℃ for austenitizing, followed by cooling; B. Tempering treatment: The austenitized sample is tempered at 650~750℃ for 20~40 minutes, and then cooled. C. Grinding, mechanical polishing, electrolytic corrosion, and light polishing: After tempering, the sample is ground with 100#~1100# sandpaper in a gradient manner, then mechanically polished, and then electrolytically corroded using an oxalic acid aqueous solution with a mass concentration of 5%~10% as the electrolyte. The sample after electrolytic corrosion is then lightly polished; the electrolytic corrosion voltage is 6~10V, the electrolytic corrosion time is 20~40 seconds, and the light polishing time is 20~30 seconds; D. Gradient Cyclic Treatment: Repeat step C for the sample after light polishing in step C, repeating 1 to 3 times. The electrolytic corrosion voltage, electrolytic corrosion time, and light polishing time are gradually reduced (i.e., steps C and D are performed 2 to 4 times in total, and the electrolytic corrosion voltage, electrolytic corrosion time, and light polishing time are gradually reduced relative to the previous one). The electrolytic corrosion voltage is gradually reduced from 6 to 10V in step C to 2 to 4V, the electrolytic corrosion time is gradually shortened from 25 to 40 seconds in step C to 10 to 15 seconds, and the light polishing time is gradually shortened from 20 to 30 seconds in step C to 5 to 10 seconds. E. The sample after gradient cyclic treatment can be examined under a metallographic microscope.

[0011] In the above-mentioned display method, in step A, the chemical composition of the Cr-Mo-V hot work die steel, in mass percentage, is: C: 0.3%~0.5%, Si: 0.3%~1.1%, Mn: 0.2%~1.0%, Cr: 4.0%~6.0%, Mo: 1.0%~3.0%, V: 0.5%~2.0%, with the balance being Fe and unavoidable impurities.

[0012] In the above-mentioned display method, in step A, the Cr-Mo-V hot work die steel is in a spheroidized annealed state, a forged state, a quenched state, or a tempered state; preferably, it is in a spheroidized annealed state.

[0013] In the above display method, in step A, the heat preservation time is 20-40 minutes.

[0014] In the above display method, step A, the cooling is either oil cooling to room temperature or water cooling to room temperature.

[0015] In the above display method, step B, the cooling is air cooling to room temperature.

[0016] Preferably, in the above display method, in step C, gradient sanding is performed using sandpaper of 100#~150#, 350#~450#, 750#~850# and 950~1100# in sequence.

[0017] In the above-mentioned display method, in step C, during electrolytic corrosion, the cathode is a stainless steel plate, the anode is the sample, and the distance between the two electrodes is fixed at 2~5cm.

[0018] In the above-mentioned display method, step C involves light polishing using diamond polishing paste.

[0019] Preferably, in the above display method, in step C, the particle size of the diamond polishing paste is 0.5~1μm.

[0020] In the above display method, in step D, when repeated once, the electrolytic corrosion voltage is 2~4V, the electrolytic corrosion time is 10~15 seconds, and the light polishing time is 5~10 seconds. When repeating the process twice, the first electrolytic etching voltage is 4~6V, the electrolytic etching time is 15~25 seconds, and the light polishing time is 10~20 seconds. The second electrolytic etching voltage is 2~4V, the electrolytic etching time is 10~15 seconds, and the light polishing time is 5~10 seconds. When repeating the process three times, the first electrolytic etching voltage is 5~6V, the electrolytic etching time is 20~25 seconds, and the light polishing time is 15~20 seconds; the second electrolytic etching voltage is 4~5V, the electrolytic etching time is 15~20 seconds, and the light polishing time is 10~15 seconds; the third electrolytic etching voltage is 2~4V, the electrolytic etching time is 10~15 seconds, and the light polishing time is 5~10 seconds.

[0021] Preferably, in the above display method, step D is repeated 2 to 3 times.

[0022] The beneficial effects of this invention are: The method of this invention does not require secondary sample preparation and the grinding process does not require maintaining a certain angle with the sandpaper to grind out the transition area, making the sample preparation process simple. It uses oxalic acid aqueous solution for electrolytic corrosion, which is easy to obtain and highly operable. The step-by-step cyclic electrolytic corrosion + light polishing method enhances the contrast difference between the grain boundaries and the matrix, which can truly preserve and more clearly display the austenite grain boundaries, and greatly avoid the problem of incomplete grain boundary display, resulting in clear and high-contrast grain boundary morphology, which is convenient for accurate grain size rating. Attached Figure Description

[0023] Figure 1 This is a metallographic image of Example 1.

[0024] Figure 2 This is a metallographic image of Example 2.

[0025] Figure 3 This is a metallographic image of Comparative Example 1.

[0026] Figure 4 The image shown is a metallographic image of Comparative Example 2. Detailed Implementation

[0027] The main chemical composition of the Cr-Mo-V hot work die steel of this invention, expressed as a percentage by mass, is as follows: C: 0.3%~0.5%, Si: 0.3%~1.1%, Mn: 0.2%~1.0%, Cr: 4.0%~6.0%, Mo: 1.0%~3.0%, V: 0.5%~2.0%, with the balance being Fe and unavoidable impurities.

[0028] In this field, it is generally difficult to characterize the grain size of spheroidized annealed steel. Therefore, this invention focuses on the sample preparation process of spheroidized annealed Cr-Mo-V hot work die steel. Thus, the method of this invention is particularly applicable to hot work die steel in the spheroidized annealed state, but it is also applicable to other states, such as forged, quenched, and tempered states. The austenite grain size display method of this invention includes pre-heat treatment and stepped cyclic electrolytic corrosion + light polishing, specifically including the following steps: Step 1: Select a suitable location on the hot work die steel to be tested and cut a metallographic sample. Place the sample in a box furnace for low-temperature (the quenching temperature of conventional hot work die steel is usually above 1000℃) short-time austenitizing treatment. The holding temperature for the austenitizing treatment is 850~950℃, and the holding time is 20~40 minutes, followed by oil cooling. Choosing 850~950℃ for a short holding time is to transform the pearlite / ferrite structure into austenite and partially dissolve the spheroidal carbides, providing conditions for subsequent carbide precipitation at grain boundaries. If the temperature is too low or the time is too short, the transformation of the structure and the dissolution of carbides will be insufficient; if the temperature is too high (such as the conventional quenching temperature) or the time is too long, there is a risk of austenite grain growth. Rapid cooling yields a martensitic structure, retaining the austenite grain boundary characteristics at high temperatures.

[0029] When cutting metallographic samples from hot work die steel to be tested, the present invention can take samples from the required testing locations, or select locations such as the center, half radius, or edge of the material cross-section to cut metallographic samples.

[0030] Step 1: After austenitizing treatment and heat preservation, oil cooling is performed to fix the original austenite grains through rapid cooling. If there is no risk of cracking in the sample, a faster cooling rate (such as water cooling) can also be used, but slower cooling rates such as air cooling cannot be used.

[0031] Step two involves tempering the sample treated in step one at 650–750°C for 20–40 minutes, followed by air cooling. During high-temperature tempering (in conventional tempering of martensitic steel, low-temperature tempering is typically 150–250°C, medium-temperature tempering is typically 350–500°C, and high-temperature tempering is typically above 500°C), martensite decomposes, leading to strong carbide precipitation kinetics. Since grain boundaries are high-energy regions, carbides preferentially nucleate and precipitate at the original austenite grain boundaries. A holding time of 20–40 minutes is sufficient for grain boundary carbides to precipitate and grow to a certain size, forming a continuous or semi-continuous chain-like distribution, thus highlighting the original austenite grain boundary outline. Excessive holding time can lead to excessive carbide growth or spheroidization, reducing grain boundary contrast; insufficient holding time results in incomplete precipitation and incomplete grain boundary display.

[0032] Step two: After tempering and heat preservation, slow cooling is required to allow sufficient time for the carbides precipitated at the grain boundaries during the heat preservation process to further precipitate, aggregate, and coarsen, thereby enhancing the grain boundary contrast. Therefore, air cooling or a slower cooling rate can be used, but oil cooling or water cooling cannot be used.

[0033] Step 3: The pre-heat-treated sample is sequentially polished using 100#~1100# sandpaper (using 100#~150#, 350#~450#, 750#~850#, and 950~1100# sandpaper, preferably 120#, 400#, 800#, and 1000# sandpaper) from smallest to largest, until the surface scratches are uniform and consistent in direction. Then, mechanical polishing is performed to obtain a scratch-free mirror finish. Next, electrolytic corrosion is carried out using oxalic acid aqueous solution as the electrolyte. The cathode is a stainless steel plate, and the anode is the sample, with the distance between the two electrodes maintained at 2~5 cm. After pre-heat treatment, the chain carbides enriched at the grain boundaries and the surrounding carbon-depleted regions have a potential difference with the matrix within the grains; under the action of an electric field, the grain boundary region is preferentially corroded. Oxalic acid solution exhibits a moderate corrosion rate and good selectivity for carbide and ferrite matrices, clearly revealing the original austenite grain boundaries while preventing excessive corrosion and blackening of the matrix. The oxalic acid aqueous solution concentration is 5%–10% (mass fraction), the voltage is 6–10V, and the corrosion time is 25–40 seconds. The initial corrosion, using a higher voltage and longer corrosion time, provides a stronger driving force, enabling rapid and relatively deep electrolytic corrosion across the entire sample surface, forming relatively deep corrosion grooves at the austenite grain boundaries. After electrolytic corrosion, the sample is rinsed with water and dried. Then, the deeply electrolytically corroded sample is lightly polished for 20–30 seconds on a fine-fiber polishing cloth (such as silk) using a fine-grained diamond polishing paste (e.g., 0.5–1 μm). The polishing force should be gentle, using only the sample's own weight. Light polishing removes corrosion products formed in the intragranular regions during electrolytic corrosion. Since the grooves at the grain boundaries are deeper, light polishing is insufficient to completely remove them. Ultimately, the intracrystalline region is repolished to a bright, flat, and clean state, while the grain boundary grooves are partially preserved.

[0034] Step four: Based on the observation results under a metallographic microscope, repeat step three one to three times, usually two to three times. The electrolysis and light polishing parameters are gradually decreased: the electrolysis voltage is gradually reduced from 6-10V to 2-4V, the electrolysis time from 20-40 seconds to 10-15 seconds, and the light polishing time from 20-30 seconds to 5-10 seconds. The milder corrosion conditions result in greater selectivity, acting almost exclusively on previously formed, higher-energy grain boundary grooves, further deepening them. The impact on the polished, lower-energy intragranular regions is minimal, thus preventing further over-corrosion of these regions. Through multiple cycles, the contrast between grain boundaries and intragranular areas is continuously amplified, aiding in subsequent grain boundary observation and grain size assessment.

[0035] Specifically, in step four, when repeated once, the electrolytic etching voltage is 2-4V, the electrolytic etching time is 10-15 seconds, and the light polishing time is 5-10 seconds; when repeated twice, the first electrolytic etching voltage is 4-6V, the electrolytic etching time is 15-25 seconds, and the light polishing time is 10-20 seconds; the second electrolytic etching voltage is 2-4V, the electrolytic etching time is 10-15 seconds, and the light polishing time is 5-10 seconds; when repeated three times, the first electrolytic etching voltage is 5-6V, the electrolytic etching time is 20-25 seconds, and the light polishing time is 15-20 seconds; the second electrolytic etching voltage is 4-5V, the electrolytic etching time is 15-20 seconds, and the light polishing time is 10-15 seconds; the third electrolytic etching voltage is 2-4V, the electrolytic etching time is 10-15 seconds, and the light polishing time is 5-10 seconds.

[0036] The present invention will be further described in detail below through embodiments, but the scope of protection of the present invention is not limited to the embodiments described herein.

[0037] In the examples and comparative examples, both test specimen 1 and test specimen 2 were sampled from the central region of the cross-section of the material, and the material was in the spheroidized annealed state.

[0038] Example 1 Step 1: Place the sample to be tested 1 (C: 0.32%, Si: 0.80%, Mn: 0.30%, Cr: 5.40%, Mo: 1.80%, V: 1.00%) in a box furnace and keep it at 900℃ for 30 minutes. Then take it out and oil cool it to room temperature.

[0039] Step 2: The sample treated in Step 1 is kept at 730℃ for 30 minutes, and then air-cooled to room temperature.

[0040] Step 3: The pre-heat-treated sample was successively polished with 120#, 400#, 800#, and 1000# sandpaper, followed by mechanical polishing to obtain a scratch-free mirror finish. Oxalic acid aqueous solution was used as the electrolyte for electrolytic corrosion. The cathode was a stainless steel plate, and the anode was the sample, with a fixed distance of 3 cm between the electrodes. The oxalic acid aqueous solution concentration was 10% (mass fraction), the voltage was 8V, and the corrosion time was 35 seconds. After electrolytic corrosion, the sample was rinsed with water and dried. Then, the deeply electrolytically corroded sample was lightly polished on silk for 25 seconds using diamond polishing paste with a particle size of 0.5μm.

[0041] Step four, repeat step three twice. The electrolysis voltages are 6V and 4V respectively, the electrolysis times are 25 seconds and 15 seconds respectively, and the light polishing times are 15 seconds and 10 seconds respectively. Finally, as... Figure 1 As shown in the 50μm scale bar, the austenite grain boundaries are clearly and accurately displayed.

[0042] Example 2 Step 1: Place the sample 2 to be tested (C: 0.38%, Si: 0.95%, Mn: 0.50%, Cr: 5.20%, Mo: 1.40%, V: 0.90%) in a box furnace and keep it at 870℃ for 35 minutes. Then take it out and oil cool it to room temperature.

[0043] Step 2: The sample treated in Step 1 is kept at 750℃ for 25 minutes, and then air-cooled to room temperature.

[0044] Step 3: The pre-heat-treated sample was successively polished with 120#, 400#, 800#, and 1000# sandpaper, followed by mechanical polishing to obtain a scratch-free mirror finish. Oxalic acid aqueous solution was used as the electrolyte for electrolytic corrosion. The cathode was a stainless steel plate, and the anode was the sample, with a fixed distance of 4 cm between the electrodes. The oxalic acid aqueous solution concentration was 10% (mass fraction), the voltage was 8V, and the corrosion time was 40 seconds. After electrolytic corrosion, the sample was rinsed with water and dried. Then, the deeply electrolytically corroded sample was lightly polished on silk for 30 seconds using diamond polishing paste with a particle size of 0.5μm.

[0045] Step four, repeat step three twice. The electrolysis voltages are 5V and 3V respectively, the electrolysis times are 25 seconds and 10 seconds respectively, and the light polishing times are 20 seconds and 5 seconds respectively. Finally, as... Figure 2 As shown in the 50μm scale bar, the austenite grain boundaries are clearly and accurately displayed.

[0046] Comparative Example 1 The pre-heat treatment method for sample 1 in Comparative Example 1 is the same as that in Example 1. However, the corrosion method used in step three is different from that in Example 1, and step four is omitted. The details are as follows: Step 1: Place the sample to be tested 1 in a box furnace and keep it at 900℃ for 30 minutes, then take it out and cool it to room temperature with oil.

[0047] Step 2: The sample treated in Step 1 is kept at 730℃ for 30 minutes, and then air-cooled to room temperature.

[0048] Step three: After pre-heat treatment, the sample is successively polished with 120#, 400#, 800#, and 1000# sandpaper, followed by mechanical polishing to obtain a scratch-free mirror finish. The sample is then subjected to electrolytic etching using supersaturated picric acid and sodium dodecylbenzenesulfonate (corrosion inhibitor) at 60℃ for 15 seconds. After electrolytic etching, the sample is rinsed with water and dried. Finally, as shown... Figure 3 As shown in the scale bar (50 μm), the matrix morphology is obvious, and the contrast with the austenite grain boundaries is basically the same, making it impossible to clearly show the original austenite grain boundaries of the sample.

[0049] Comparative Example 2 Comparative Example 2, Test Sample 2, lacked step four, the step-by-step cyclic electrolytic corrosion + light polishing. The remaining steps were the same as in Example 2, as follows: Step 1: Place the sample 2 to be tested in a box furnace and keep it at 870°C for 35 minutes, then take it out and cool it to room temperature with oil.

[0050] Step 2: The sample treated in Step 1 is kept at 750℃ for 25 minutes, and then air-cooled to room temperature.

[0051] Step 3: The pre-heat-treated sample was successively polished with 120#, 400#, 800#, and 1000# sandpaper, followed by mechanical polishing to obtain a scratch-free mirror finish. Oxalic acid aqueous solution was used as the electrolyte for electrolytic corrosion. The cathode was a stainless steel plate, and the anode was the sample, with a fixed distance of 4 cm between the electrodes. The oxalic acid aqueous solution concentration was 10% (mass fraction), the voltage was 8V, and the corrosion time was 40 seconds. After electrolytic corrosion, the sample was rinsed with water and dried. Then, the deeply electrolytically corroded sample was lightly polished on silk for 30 seconds using diamond polishing paste with a particle size of 0.5μm. Finally, as shown... Figure 4 As shown in the 50 μm scale bar, the clarity of the austenite grain boundaries is worse compared to Example 2.

Claims

1. A method for displaying the austenitic grain size of Cr-Mo-V series hot work die steel, characterized in that: Includes the following steps: A. Austenitizing treatment: The Cr-Mo-V series hot work die steel test samples were subjected to heat treatment at 850~950℃ for austenitizing, followed by cooling; B. Tempering treatment: The austenitized sample is tempered at 650~750℃ for 20~40 minutes, and then cooled. C. Grinding, mechanical polishing, electrolytic corrosion and light polishing: The tempered sample is ground with 100#~1100# sandpaper in a gradient, then mechanically polished, and then electrolytically corroded using an oxalic acid aqueous solution with a mass concentration of 5%~10% as the electrolyte. The sample after electrolytic corrosion is then lightly polished. The electrolytic corrosion voltage is 6~10V, the electrolytic corrosion time is 20~40 seconds, and the light polishing time is 20~30 seconds; D. Gradient cyclic treatment: Repeat step C for the sample after light polishing in step C, repeating 1 to 3 times. The electrolytic corrosion voltage, electrolytic corrosion time and light polishing time are gradually reduced. The electrolytic corrosion voltage is gradually reduced from 6 to 10V in step C to 2 to 4V, the electrolytic corrosion time is gradually shortened from 25 to 40 seconds in step C to 10 to 15 seconds, and the light polishing time is gradually shortened from 20 to 30 seconds in step C to 5 to 10 seconds. E. The sample after gradient cyclic treatment can be examined under a metallographic microscope.

2. The method for displaying austenite grain size in Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step A, the chemical composition of the Cr-Mo-V hot work die steel, in mass percentage, is: C: 0.3%~0.5%, Si: 0.3%~1.1%, Mn: 0.2%~1.0%, Cr: 4.0%~6.0%, Mo: 1.0%~3.0%, V: 0.5%~2.0%, with the balance being Fe and unavoidable impurities.

3. The method for displaying austenite grain size in Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step A, the Cr-Mo-V hot work die steel is in the spheroidized annealed state, forged state, quenched state, or tempered state.

4. The method for displaying the austenite grain size of Cr-Mo-V series hot work die steel according to claim 1, characterized in that: At least one of the following must be met: In step A, the heat preservation time is 20-40 minutes; In step A, the cooling is either oil cooling to room temperature or water cooling to room temperature.

5. The method for displaying the austenite grain size of Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step B, the cooling is air cooling to room temperature.

6. The method for displaying the austenite grain size of Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step C, use sandpaper of 100#~150#, 350#~450#, 750#~850# and 950~1100# for gradient sanding in sequence.

7. The method for displaying austenite grain size in Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step C, during electrolytic corrosion, the cathode is a stainless steel plate, the anode is the sample, and the distance between the two electrodes is kept constant at 2~5cm.

8. The method for displaying the austenite grain size of Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step C, a light polish is performed using diamond polishing paste.

9. The method for displaying the austenite grain size of Cr-Mo-V series hot work die steel according to claim 1, characterized in that: The diamond polishing paste has a particle size of 0.5~1μm.

10. The method for displaying the austenite grain size of Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step D, when repeated once, the electrolytic corrosion voltage is 2~4V, the electrolytic corrosion time is 10~15 seconds, and the light polishing time is 5~10 seconds; When repeating the process twice, the first electrolytic etching voltage is 4~6V, the electrolytic etching time is 15~25 seconds, and the light polishing time is 10~20 seconds. The second electrolytic etching voltage is 2~4V, the electrolytic etching time is 10~15 seconds, and the light polishing time is 5~10 seconds. When repeating the process three times, the first electrolytic etching voltage is 5~6V, the electrolytic etching time is 20~25 seconds, and the light polishing time is 15~20 seconds; the second electrolytic etching voltage is 4~5V, the electrolytic etching time is 15~20 seconds, and the light polishing time is 10~15 seconds; the third electrolytic etching voltage is 2~4V, the electrolytic etching time is 10~15 seconds, and the light polishing time is 5~10 seconds.

11. The method for displaying the austenite grain size of Cr-Mo-V series hot work die steel according to claim 1, characterized in that: In step D, repeat 2 to 3 times.

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