Method for precisely distinguishing multiphase structure of w780x hot-rolled plate

Abstract

The present application relates to the technical field of metallographic analysis detection, and specifically discloses a display method for accurately distinguishing the multiphase structure of W780X hot-rolled plate, which comprises wire cutting sampling, hot inlaying, grinding, polishing, and etching; the polished sample is soaked in a sodium thiosulfate solution for 10-200 seconds, and then the surface is wiped with a hydrochloric acid alcohol cotton ball to remove the surface corrosion products, washed with alcohol, and dried; metallographic microscope observation can clearly show the morphology, size and distribution state of ferrite, pearlite and martensite. Compared with the prior art, the etchant used in the present application is a solution prepared from 8-15 g of sodium thiosulfate and 77-90 ml of water, and after etching, the three kinds of structures can be clearly shown, the cost is low, it is safe and non-toxic, and it is convenient to prepare; the morphology and distribution state of ferrite, pearlite and martensite in the metallographic structure of the W780X hot-rolled plate can be clearly and completely shown; and the method has the characteristics of low detection cost, rapidness, simplicity and accuracy.

Description

Technical Field

[0001] This invention relates to the field of metallographic analysis and testing technology, and in particular to a display method for accurately distinguishing the multiphase microstructure of W780X hot-rolled steel plate. Background Technology

[0002] During the production of hot-rolled duplex steel W780X, abnormal performance fluctuations occurred at the tail end. Severe tail-end jumping occurred during cold rolling, leading to production shutdowns. Analysis revealed that the microstructure of the hot-rolled W780X was ferrite + pearlite in the middle section, while the tail section consisted of ferrite + pearlite + martensite. To improve the process, the microstructure at the tail end must be clearly visualized to correctly identify problems in the hot-rolling cooling process. However, traditional nitric acid-alcohol etching cannot clearly observe the martensite in the microstructure, thus failing to provide direction for process improvement. Traditional etching agents for martensite in duplex steel use picric acid coloring, but picric acid is an explosive, flammable, irritating, and toxic substance, and its use is restricted and its procurement is extremely difficult.

[0003] Patent CN112763292A discloses a method for displaying the microstructure of ferritic-martensitic dual-phase steel, introducing a method applicable to displaying martensite in ferritic-martensitic dual-phase steel. However, this method is applicable to ferritic + martensitic dual-phase structures, while the microstructure of the tail section of hot-rolled W780X steel is ferritic + pearlite + martensite. The method disclosed in patent CN112763292A cannot distinguish between martensite and pearlite, treating pearlite in the microstructure as martensite along with the martensite, or coloring pearlite black. It cannot accurately distinguish between ferritic, pearlite, and martensite in the microstructure. Therefore, a new detection method is needed that can clearly display the martensitic microstructure in the tail section of hot-rolled W780X steel, providing direction for process improvement. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for accurately distinguishing the multiphase microstructure of W780X hot-rolled steel plates. It abandons traditional nitric acid-alcohol etching and picric acid coloring methods, employing an innovative etching method that allows for simple, rapid, and low-cost display and observation of martensite, ferrite, and pearlite in the metallographic structure of W780X hot-rolled steel plates. It clearly displays the morphology, size, and distribution of each phase, directly providing direction for process improvement.

[0005] To achieve this technical objective, the present invention adopts the following solution:

[0006] A method for accurately distinguishing the multiphase microstructure of W780X hot-rolled steel sheet includes the following steps:

[0007] S1. Sampling: Use wire cutting to cut samples of the full thickness of the plate to be tested;

[0008] S2, Embedding: Place the sample from step S1 with the rolling direction facing downwards into the hot embedding machine for hot embedding;

[0009] S3. Grinding: Grind the sample embedded in step S2.

[0010] S4. Polishing: Polish the sample that has been ground in step S3.

[0011] S5. Corrosion: Immerse the polished sample from step S4 in the etchant for 10-200 seconds. The etchant is sodium thiosulfate solution. After removal, wipe the surface with a hydrochloric acid-alcohol cotton ball to remove the corrosion products. Rinse with alcohol and dry.

[0012] S6. Metallographic microscopy observation: Place the sample prepared in step S5 under a metallographic microscope for observation. Magnification can clearly show the morphology, size and distribution of ferrite, pearlite and martensite.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] 1) This invention provides a simple, fast, and low-cost method for displaying and observing the metallographic structure of W780X hot-rolled plate. It can quickly, accurately, clearly, continuously, and uniformly display and observe the structure, morphology, proportion, size, and distribution of ferrite, pearlite, and martensite in the metallographic structure of W780X hot-rolled plate, providing a basis for process improvement and quality enhancement.

[0015] 2) The method of this invention breaks away from the traditional picric acid staining method. The etchant used is a solution prepared by 8~15 g sodium thiosulfate and 77~90 ml water. The corrosion rate is easy to control. The etchant is low in cost, safe and non-toxic, and easy to prepare. It can clearly and completely display the microstructure of the metallographic structure of W780X hot-rolled plate.

[0016] 3) The method provided by this invention has the characteristics of low detection cost, speed, simplicity and accuracy.

[0017] Furthermore, a preferred embodiment of the present invention is as follows:

[0018] A method for accurately distinguishing the multiphase microstructure of W780X hot-rolled steel sheet includes the following steps:

[0019] S1. Sampling: Use wire cutting to cut samples of the full thickness of the plate to be tested. The sample size is 10mm×10mm~15mm×15mm.

[0020] S2, Insertion: Place the sample from step S1 with the rolling direction facing down into the hot inserting machine for hot inserting. Use a pressure of 290 bar and hold for 4 min.

[0021] S3. Grinding: After the inlaid sample is coarsely ground on a grinding machine, it is then finely ground with 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears. The sandpaper number in this application refers to the grit number.

[0022] S4. Polishing: Polish the prepared sample using an automatic metallographic polishing machine. Spray 1.0~2.5 μm polishing agent onto a 250 mm polishing cloth and polish at a speed of 900~1400 r / min. Spray water while polishing for 1~2 minutes until the surface is bright and free of scratches.

[0023] S5. Corrosion: Immerse the polished sample in the etchant, which is a solution prepared by 8~15 g sodium thiosulfate and 77~90 ml water. Corrode for 10~200 s. After the sample surface darkens, remove it and wipe the surface 1~3 times with a cotton ball soaked in 0.1%~0.5% hydrochloric acid alcohol to remove the corrosion products on the surface. Rinse it with alcohol and dry it with a hair dryer.

[0024] S6. Metallographic microscopy observation: Place the prepared sample under a metallographic microscope for observation. Magnification can clearly show the morphology, size and distribution of ferrite, pearlite and martensite. Attached Figure Description

[0025] Figure 1 The images show the morphology of ferrite, pearlite, and martensite in the metallographic structure of the W780X hot-rolled plate in Embodiment 1 of the present invention.

[0026] Figure 2 The result of hydrochloric acid-alcohol etching in Comparative Example 1 of this invention does not clearly show the morphology of martensite;

[0027] Figure 3 The images show the morphology of ferrite, pearlite, and martensite in the metallographic structure of the W780X hot-rolled plate in Embodiment 2 of the present invention.

[0028] Figure 4 The results shown in Comparative Example 2 of this invention are obtained using the method described in patent CN112763292A;

[0029] Figure 5 This is the result of nitric acid-alcohol etching in Comparative Example 3 of the present invention;

[0030] Figure 6 The images show the morphology of ferrite, pearlite, and martensite in the metallographic structure of the W780X hot-rolled plate in Embodiment 3 of the present invention. Detailed Implementation

[0031] To fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific embodiments, but the present invention is not limited thereto.

[0032] This invention provides a method for accurately distinguishing the multiphase microstructure of W780X hot-rolled steel plate, including sampling, mounting, grinding, polishing, etching, and metallographic microscopy observation; specifically: Example 1

[0033] S1. Sampling: Take a 2.5mm thick W780X hot-rolled plate and cut a full-thickness sample using wire cutting. The sample size is 10mm×10mm.

[0034] S2, Insertion: Place the sample from step S1 with the rolling direction facing down into the hot inserting machine, apply a pressure of 290 bar, and hold for 4 minutes to perform hot inserting;

[0035] S3. Grinding: After rough grinding the inlaid sample on a grinding machine, fine grinding is carried out using 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears.

[0036] S4. Polishing: The ground sample is polished using an automatic metallographic polishing machine. 1.0μm polishing agent is sprayed onto a 250mm polishing cloth. Polishing is carried out at a speed of 1400 r / min, with water sprayed while polishing. Polishing is carried out for 2 minutes until the surface is bright and free of scratches. The polishing agent in this application is a diamond spray polishing agent, which can be any commercially available polishing agent, selected according to specific needs.

[0037] S5. Corrosion: Immerse the polished sample in the etchant, which is a chemical etchant solution prepared from 8 g of sodium thiosulfate and 90 ml of water, for 120 s. Remove the sample after the surface darkens, wipe the surface once with a cotton ball soaked in 0.5% hydrochloric acid alcohol to remove corrosion products, rinse with alcohol, and dry with a hair dryer.

[0038] S6. Metallurgical Microscopy Observation: The prepared sample was observed under a bright field ZEISS metallurgical microscope, which clearly showed the morphology, size, and distribution of each phase in the microstructure. The black areas represented pearlite, the island-like structures represented martensite, and the remaining gray areas represented ferrite. The results are shown in [Figure number missing]. Figure 1 .

[0039] Comparative Example 1: Etching with nitric acid and alcohol

[0040] S1. Sampling: Take a 2.5mm thick W780X hot-rolled plate and cut a full-thickness sample using wire cutting. The sample size is 10mm×10mm.

[0041] S2, Insertion: Place the sample from step S1 with the rolling direction facing down into the hot inserting machine, apply a pressure of 290 bar, and hold for 4 minutes to perform hot inserting;

[0042] S3. Grinding: After rough grinding the inlaid sample on a grinding machine, fine grinding is carried out using 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears.

[0043] S4. Polishing: The ground sample is polished using an automatic metallographic polishing machine. 1.0μm polishing agent is sprayed onto a 250mm polishing cloth. Polishing is carried out at a speed of 1400 r / min. Water is sprayed while polishing. Polishing is carried out for 2 minutes until the surface is bright and free of scratches.

[0044] S5. Corrosion: The polished sample is corroded with 4% nitric acid alcohol for 3 seconds, rinsed with alcohol, and dried with a hair dryer.

[0045] S6. Metallurgical Microscopy Observation: The prepared sample was observed under a metallurgical microscope. The morphology, size, and distribution of island-like martensite in the microstructure were not clearly visible. Results are shown below. Figure 2 . Example 2

[0046] S1. Sampling: Take a 3.0mm thick W780X hot-rolled plate and cut a full-thickness sample using wire cutting. The sample size is 10mm×10mm.

[0047] S2, Insertion: Place the sample from step S1 with the rolling direction facing down into the hot inserting machine, apply a pressure of 290 bar, and hold for 4 minutes to perform hot inserting;

[0048] S3. Grinding: After rough grinding the inlaid sample on a grinding machine, fine grinding is carried out using 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears.

[0049] S4. Polishing: The ground sample is polished using an automatic metallographic polishing machine. 1.0μm polishing agent is sprayed onto a 250mm polishing cloth. Polishing is carried out at a speed of 900 r / min. Water is sprayed while polishing. Polishing is carried out for 2 minutes until the surface is bright and free of scratches.

[0050] S5. Corrosion: Immerse the polished sample in the etchant, which is a chemical etchant solution prepared by 10 g sodium thiosulfate and 85 ml water, for 60 s. Remove the sample after the surface darkens, wipe the surface twice with a cotton ball soaked in 0.3% hydrochloric acid alcohol to remove the corrosion products, rinse with alcohol, and dry with a hair dryer.

[0051] S6. Metallographic Microscopy Observation: The prepared sample is observed under a metallographic microscope, which clearly displays the morphology, size, and distribution of ferrite, pearlite, and martensite in the multiphase microstructure. Black represents pearlite, island-like structures represent martensite, and the remaining gray areas represent ferrite. Results are shown in [Figure number missing]. Figure 3 .

[0052] Comparative Example 2: Corrosion was performed using the etchant described in patent CN112763292A.

[0053] S1. Sampling: Take a 3.0mm thick W780X hot-rolled plate and cut a full-thickness sample using wire cutting. The sample size is 10mm×10mm.

[0054] S2, Insertion: Place the sample from step S1 with the rolling direction facing down into the hot inserting machine, apply a pressure of 290 bar, and hold for 4 minutes to perform hot inserting;

[0055] S3. Grinding: After rough grinding the inlaid sample on a grinding machine, fine grinding is carried out using 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears.

[0056] S4. Polishing: The ground sample is polished using an automatic metallographic polishing machine. 1.0μm polishing agent is sprayed onto a 250mm polishing cloth. Polishing is carried out at a speed of 900 r / min. Water is sprayed while polishing. Polishing is carried out for 2 minutes until the surface is bright and free of scratches.

[0057] S5. Corrosion: Prepare 100 ml of saturated sodium thiosulfate aqueous solution at room temperature, and add 3 g of sodium metabisulfite and 1 ml of nitric acid to the sodium thiosulfate aqueous solution. Place the polished sample from step S4 into the corrosion solution and corrode until the surface turns blue-black. Then rinse with running water and blow dry.

[0058] S6. Metallographic Microscope Observation: The prepared sample was observed under a ZEISS metallographic microscope in bright field. Only two microstructures were observed after corrosion: black and white. Black represents ferrite, and white represents martensite. The results are shown below. Figure 4 This method mistakenly etches some pearlite into martensite or ferrite, making it unsuitable for distinguishing multiphase structures. The reason is that the etchant is a composite etchant composed of sodium metabisulfite, sodium thiosulfate, and nitric acid. Nitric acid provides strong oxidizing properties to rapidly etch the structure, sodium thiosulfate helps dissolve corrosion products and buffers the oxidation intensity, and sodium metabisulfite adjusts the acidity and reducing properties. The combined effect of these three substances makes the corrosion too fast and leads to structural distortion.

[0059] Comparative Example 3: Etching with nitric acid and alcohol

[0060] S1. Sampling: Take a 3.0mm thick W780X hot-rolled plate and cut a full-thickness sample using wire cutting. The sample size is 10mm×10mm.

[0061] S2, Insertion: Place the sample from step S1 with the rolling direction facing down into the hot inserting machine, apply a pressure of 290 bar, and hold for 4 minutes to perform hot inserting;

[0062] S3. Grinding: After rough grinding the inlaid sample on a grinding machine, fine grinding is carried out using 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears.

[0063] S4. Polishing: The ground sample is polished using an automatic metallographic polishing machine. 1.0μm polishing agent is sprayed onto a 250mm polishing cloth. Polishing is carried out at a speed of 900 r / min. Water is sprayed while polishing. Polishing is carried out for 2 minutes until the surface is bright and free of scratches.

[0064] S5. Etching: Etch the polished sample with 4% nitric acid alcohol for 3-5 seconds, rinse with alcohol, and blow dry.

[0065] S6. Metallographic Microscope Observation: The prepared sample was observed under a ZEISS metallographic microscope in bright field. After etching, only ferrite and pearlite were visible; martensite was not. The results are shown in [Figure number missing]. Figure 5 A comparison of Example 2, Comparative Example 2, and Comparative Example 3 revealed that the method of this patent can accurately distinguish ferrite, pearlite, and martensite in the tissue. Example 3

[0066] S1. Sampling: Take a 3.5mm thick W780X hot-rolled plate and cut a full-thickness sample using wire cutting. The sample size is 10mm×10mm.

[0067] S2, Insertion: Place the sample from step S1 with the rolling direction facing down into the hot inserting machine, apply a pressure of 290 bar, and hold for 4 minutes to perform hot inserting;

[0068] S3. Grinding: After rough grinding the inlaid sample on a grinding machine, fine grinding is carried out using 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears.

[0069] S4. Polishing: The ground sample is polished using an automatic metallographic polishing machine. 2.5μm polishing agent is sprayed onto a 250mm polishing cloth. Polishing is carried out at a speed of 1400 r / min. Water is sprayed while polishing. Polishing is carried out for 1 minute until the surface is bright and free of scratches.

[0070] S5. Etching: Immerse the polished sample in the etching agent, which is a solution prepared by 15 g sodium thiosulfate and 77 ml water, for 10 seconds. Remove the sample after the surface darkens, wipe the surface three times with a cotton ball soaked in 0.1% hydrochloric acid alcohol to remove the corrosion products, rinse with alcohol, and dry with a hair dryer.

[0071] S6. Metallographic Microscopy Observation: The prepared sample was observed under a metallographic microscope. Ferrite, pearlite, and martensite in the multiphase microstructure were clearly visible. Black represents pearlite, island-like structures represent martensite, and the remaining gray areas represent ferrite. Results are shown below. Figure 6 .

[0072] This invention utilizes sodium thiosulfate to selectively oxidize and corrode different phases by coordinating and dissolving free metal ions and regulating the redox balance. Strict control of the sodium thiosulfate concentration is crucial; excessively high concentrations lead to over-dissolution of corrosion products and indistinct coloring, while insufficient concentrations fail to suppress over-oxidation, resulting in excessively dark colors. After corrosion, the surface is wiped with a cotton ball soaked in hydrochloric acid and alcohol to remove corrosion products, reducing microstructural distortion caused by "phase-selective corrosion" and ensuring clear differentiation of phase boundaries.

[0073] Finally, it should be noted that the above-listed embodiments are merely preferred embodiments of the present invention. Of course, those skilled in the art can make modifications and variations to the present invention. If such modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should be considered as being within the protection scope of the present invention.

Claims

1. A method for accurately distinguishing the display of the multiphase structure of a W780X hot-rolled plate, characterized by, Includes the following steps: S1. Sampling: Use wire cutting to cut samples of the full thickness of the plate to be tested; S2, Embedding: Place the sample from step S1 with the rolling direction facing downwards into the hot embedding machine for hot embedding; S3. Grinding: Grind the sample embedded in step S2. S4. Polishing: Polish the sample that has been ground in step S3. S5. Corrosion: Immerse the polished sample from step S4 in the etchant, which is a solution prepared by 8-15 g sodium thiosulfate and 77-90 ml water. Corrode for 10-200 s. After the sample surface darkens, remove it and wipe the surface 1-3 times with a cotton ball soaked in 0.1%-0.5% hydrochloric acid alcohol to remove the corrosion products on the surface. Rinse it with alcohol and dry it with a hair dryer. S6. Metallographic microscopy observation: Place the sample prepared in step S5 under a metallographic microscope for observation. Magnification can clearly show the morphology, size and distribution of ferrite, pearlite and martensite.

2. The method of claim 1, wherein the method is used to distinguish the multi-phase structure of a W780X hot-rolled plate. In step S1, the sample size is 10mm×10mm~15mm×15mm.

3. The method of claim 1, wherein the method is used to distinguish the multi-phase structure of a W780X hot-rolled plate. In step S2, the hot embedding is performed at a pressure of 290 bar and held at that temperature for 4 minutes.

4. The method of claim 1, wherein the method is used to distinguish the multi-phase structure of a W780X hot-rolled plate. The specific operation of step S3 is as follows: After the inlaid sample is coarsely ground on a grinding machine, it is then finely ground with 280-400-600 grit sandpaper in sequence. The sample is rotated 90° for each grinding pass until the previous scratch disappears.

5. The method of claim 1, wherein the method is used to distinguish the multi-phase structure of a W780X hot-rolled plate. The specific operation of step S4 is as follows: polish the prepared sample using an automatic metallographic polishing machine, spray 1.0~2.5 μm polishing agent on a 250 mm polishing cloth, polish at a speed of 900~1400 r / min, spray water while polishing, polish for 1~2 min until the surface is bright and free of scratches.

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