Method for rapidly distinguishing pseudo-eutectoid or recarburization structure on surface of low and medium carbon steel

Abstract

The invention relates to a method for rapidly distinguishing a pseudo-eutectoid or recarburization structure on the surface of medium-low carbon steel. The method comprises the following steps: sampling the medium-low carbon steel with an abnormal structure (pseudo-eutectoid or recarburization structure) on the surface; putting the taken sample into a heat treatment furnace for specific anti-oxidation heat treatment; transversely grinding, polishing and corroding the sample subjected to heat treatment, and then observing the prepared sample through an optical microscope; and judging whether the abnormal tissue of the sample is a pseudo-eutectoid tissue or a recarburized tissue by observing the change of the tissue of the sample. According to the method, whether the abnormal structure of the medium and low-carbon steel is the pseudo-eutectoid structure or the recarburization structure can be rapidly distinguished, an improvement direction is provided for production, and the production efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metal material microstructure detection, in particular to a method for quickly distinguishing pseudo-eutectoid or carburized structures on the surface of medium and low carbon steel. Background Art

[0002] In the Fe-Fe3C phase diagram, hypoeutectoid steel or hypereutectoid steel near the eutectoid composition (0.77% C) is cooled at an appropriately fast rate to the shaded area (called pseudo-eutectoid area) composed of the reverse extension lines of the GS line and ES line of the equilibrium diagram, so that the proeutectoid phase (proeutectoid ferrite or proeutectoid cementite) does not have time to precipitate, thus forming a pearlite-shaped eutectoid structure. Because its alloy composition is not a eutectoid composition, and the relative content of ferrite and cementite in the structure is also different from that of pearlite with a eutectoid composition, it changes with the carbon content of austenite. This transformation is called pseudo-eutectoid transformation, and its transformation product is called pseudo-eutectoid structure.

[0003] The primary cause of carbon increase is the carbon-rich layer of mold slag. This layer can contain up to six times the carbon content of the original slag and is non-sinterable. When the slag layer is thin and fluctuating, it can easily come into contact with the molten steel and even slag roll-up, causing carbon increase in the molten steel. It is believed that as long as the slag layer is maintained at a sufficient thickness (10-15 mm) under stable casting conditions and the free carbon content in the original slag is only around 1%, the mold slag will not increase carbon in the molten steel. This problem is not particularly prominent when the carbon content of the molten steel is typically less than 0.02%. However, as the required carbon content in steel decreases, especially for some ultra-pure steels requiring a carbon content of less than 0.003%, carbon increase in the molten steel occurs to some extent during continuous casting, even under normal operating conditions. Currently, domestic carbon increase levels are generally around 0.001%, while advanced international manufacturers can control it within the 0.0003% range. Therefore, addressing this issue is crucial.

[0004] For medium and low carbon steels, the composition of the pseudo-eutectoid structure is not significantly different from the material itself; it is only due to the cooling rate that the difference in structure is formed. Due to the rapid cooling rate at the specimen edge, the pro-eutectoid ferrite has no time to precipitate, resulting in the pseudo-eutectoid structure. Although the hardness and strength of the pseudo-eutectoid structure may differ from the normal structure of the specimen, the difference is not significant. However, carburization is different. Carburization can be defined as a segregation of composition, resulting in hardness and strength far exceeding the normal structure of the specimen, resulting in uneven performance of the specimen. During the inspection process, it is often difficult to distinguish between pseudo-eutectoid and carburized structures. Because the metallographic structure formed by 0.60% ≤ C ≤ 1.00% is indistinguishable from the pseudo-eutectoid structure from a normal observation, it is difficult to identify the correct direction for subsequent process improvements. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for quickly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel, which can distinguish pseudo-eutectoid structure from carburized structure, provide improved direction for production, and improve production efficiency.

[0006] The technical solution adopted by the present invention to solve the technical problem is: a method for quickly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel, comprising the following steps:

[0007] S1. Sampling of rolled materials with abnormal surface structures: cutting and intercepting samples according to the sampling requirements specified in the standard;

[0008] S2. Place the cut specimens in a heat treatment furnace for specific anti-oxidation heat treatment: After placing the heat treatment specimens in the heat treatment furnace, exhaust the air in the furnace until the pressure gauge shows 80KPa. The specimens are heated to 850℃~870℃ with the furnace. While heating, nitrogen is added as an inert protective atmosphere until the pressure gauge shows 101KPa. After the temperature reaches 101KPa, the specimens are kept in the furnace for 20min~25min, and then taken out of the furnace for air cooling.

[0009] S3. Prepare the sample after heat treatment: the cross section is subjected to the steps of rough grinding, fine grinding, polishing and etching in sequence;

[0010] S4. Place the eroded sample under an optical microscope to observe its structural changes, so as to distinguish whether the abnormal structure at the edge of the sample is a pseudo-eutectoid structure or a carburized structure.

[0011] Furthermore, in step S1 of the present invention, a grinding wheel cutter is used to prepare the sample, and the cutting fluid is kept to cool the sample at all times during the sample processing process.

[0012] Furthermore, in step S2 of the present invention, the air in the furnace is discharged to a pressure gauge indicating 80 KPa, which is a low vacuum state.

[0013] Furthermore, in step S2 of the present invention, the heat treatment temperature is 850° C. to 870° C., and the furnace is kept warm for 20 min to 25 min.

[0014] Furthermore, in step S2 of the present invention, the temperature is raised while adding an inert protective atmosphere of nitrogen until the pressure gauge shows 101 KPa.

[0015] Furthermore, in step S2 of the present invention, the sample is air-cooled after being taken out of the furnace, and natural cooling is used to ensure the precipitation of the ferrite structure of the sample.

[0016] Furthermore, in step S3 of the present invention, the heat-treated sample is coarse-ground by a spectral grinding machine, and then fine-ground by 180-mesh, 600-mesh, and 1000-mesh metallographic sandpaper in sequence, and finally polished. The entire process requires cooling with water to prevent the ground surface of the sample from overheating.

[0017] Furthermore, in step S3 of the present invention, the etchant is a 2% nitric acid alcohol solution, and the polished surface of the sample is immersed in the etchant for 19 seconds to 21 seconds. The sample is slightly shaken during the etch process to maintain etch uniformity.

[0018] The beneficial effect of the present invention is that it solves the defects existing in the background technology. The present invention samples medium and low carbon steel with abnormal structure (pseudo-eutectoid or carburized structure) on the surface, and then performs anti-oxidation heat treatment on the sample, and strictly controls the oxidation generated during the heat treatment, which can restore the pseudo-eutectoid structure to the original normal structure effect, while the carburized structure cannot be restored, and the structure can be effectively distinguished, which facilitates the identification of the sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 (a) and (b) are photos of abnormal metallographic structures at the edge of medium carbon steel; (the left is a photo of pseudo-eutectoid structure, and the right is a photo of carburized structure)

[0020] Figure 2 (a) and (b) are respectively metallographic photographs of the abnormal structure after anti-oxidation heat treatment observed under a microscope in Example 1 of the present invention; (the left one is a photograph of the pseudo-eutectoid structure after heat treatment, and the right one is a photograph of the carburized structure after heat treatment).

[0021] Figure 3 (a) and (b) are respectively metallographic photographs of the abnormal structure after anti-oxidation heat treatment observed under a microscope in Example 2 of the present invention; (the left one is a photograph of the pseudo-eutectoid structure after heat treatment, and the right one is a photograph of the carburized structure after heat treatment).

[0022] Figure 4 (a) and (b) are metallographic photographs of abnormal structures after modulated heat treatment of the comparative example of the present invention observed under a microscope; (the left one is a photograph of the pseudo-eutectoid structure after heat treatment, and the right one is a photograph of the carburized structure after heat treatment). DETAILED DESCRIPTION

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and preferred embodiments. These drawings are simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, and therefore only show the components related to the present invention.

[0024] A method for quickly distinguishing pseudo-eutectoid or carburized structures on the surface of medium- and low-carbon steels. The technical principle of this method is that pseudo-eutectoid structures are formed due to abnormal cooling rate, and heat treatment can eliminate the non-equilibrium state; carburized structures are component segregation, and heat treatment cannot change the carbon content, thus achieving differentiation.

[0025] The present invention is described in detail below with reference to specific embodiments.

[0026] Example 1

[0027] S1. Sampling of rolled materials with abnormal surface structures: cutting and intercepting samples in accordance with the sampling requirements specified in GB / T 13298-2015 "Methods for Examination of Metal Microstructures".

[0028] S2. Place the cut specimens in a heat treatment furnace for specific anti-oxidation heat treatment. After the heat treatment specimens are placed in the heat treatment furnace, exhaust the air in the furnace until the pressure gauge shows 80KPa. The specimens are heated to 850°C with the furnace. While heating, add inert protective atmosphere nitrogen until the pressure gauge shows 101KPa. After the temperature is raised, keep the temperature in the furnace for 25 minutes and then take them out of the furnace for air cooling.

[0029] S3. Prepare the sample after heat treatment: the cross section is subjected to rough grinding → fine grinding → polishing → erosion.

[0030] S4. Place the eroded sample under an optical microscope to observe its structural changes, so as to distinguish whether the abnormal structure at the edge of the sample is a pseudo-eutectoid structure or a carburized structure.

[0031] Specifically, in step S1, a grinding wheel cutter is used to cut the sample. During the sample processing process, the cutting fluid must be kept to cool the sample at all times to prevent the sample from being burned. In step S3, water must be used for cooling throughout the entire process to prevent the grinding surface of the sample from overheating.

[0032] Example 2

[0033] S1. Sampling of rolled materials with abnormal surface structures: cutting and intercepting samples in accordance with the sampling requirements specified in GB / T 13298-2015 "Methods for Examination of Metal Microstructures".

[0034] S2. Place the cut specimens in a heat treatment furnace for specific anti-oxidation heat treatment. After the heat treatment specimens are placed in the heat treatment furnace, exhaust the air in the furnace until the pressure gauge shows 80KPa. The specimens are heated to 870°C with the furnace. While heating, add inert protective atmosphere nitrogen until the pressure gauge shows 101KPa. After the temperature is raised, keep the temperature in the furnace for 20 minutes and then take them out of the furnace for air cooling.

[0035] S3. Prepare the sample after heat treatment: the cross section is subjected to rough grinding → fine grinding → polishing → erosion.

[0036] S4. Place the eroded sample under an optical microscope to observe its structural changes, so as to distinguish whether the abnormal structure at the edge of the sample is a pseudo-eutectoid structure or a carburized structure.

[0037] Specifically, in step S1, a grinding wheel cutter is used to cut the sample. During the sample processing process, the cutting fluid must be kept to cool the sample at all times to prevent the sample from being burned. In step S3, water must be used for cooling throughout the entire process to prevent the grinding surface of the sample from overheating.

[0038] Comparative Example 1

[0039] S1. Sampling of rolled materials with abnormal surface structures: cutting and intercepting samples in accordance with the sampling requirements specified in GB / T 13298-2015 "Methods for Examination of Metal Microstructures".

[0040] S2. Place the cut specimens in a heat treatment furnace for specific anti-oxidation heat treatment. After the heat treatment specimens are placed in the heat treatment furnace, they are heated to 870°C with the furnace, kept warm for 20 minutes, taken out for water cooling, heated to 550°C with the furnace, kept warm for 60 minutes, and taken out for air cooling.

[0041] S3. Prepare the sample after heat treatment: the cross section is subjected to rough grinding → fine grinding → polishing → erosion.

[0042] S4. Place the eroded sample under an optical microscope to observe its structural changes, so as to distinguish whether the abnormal structure at the edge of the sample is a pseudo-eutectoid structure or a carburized structure.

[0043] In summary, the heat treatment processes of Examples 1 and 2 (heating to 850-870°C under an inert nitrogen atmosphere followed by air cooling) effectively and clearly distinguish abnormal surface structures (pseudo-eutectoid or carburized structures), while the process of Comparative Example 1 (no protective atmosphere, water cooling followed by secondary tempering) results in blurred structural changes and is unable to reliably distinguish abnormal structure types. This demonstrates that the anti-oxidation protective atmosphere and air cooling are key factors in ensuring accurate microstructural observation.

[0044] The above description only describes specific embodiments of the present invention. Various examples do not limit the essential content of the present invention. After reading the description, ordinary technicians in the relevant technical field can make modifications or variations to the specific embodiments described above without departing from the essence and scope of the invention.

Claims

1. A method for quickly distinguishing pseudo-eutectoid or carburized structures on the surface of medium and low carbon steel, characterized by: The following steps are involved: S1. Sampling of rolled materials with abnormal surface structures: cutting and intercepting samples according to the sampling requirements specified in the standard; S2. Place the cut specimens in a heat treatment furnace for specific anti-oxidation heat treatment: After placing the heat treatment specimens in the heat treatment furnace, exhaust the air in the furnace until the pressure gauge shows 80KPa. The specimens are heated to 850℃~870℃ with the furnace. While heating, nitrogen is added as an inert protective atmosphere until the pressure gauge shows 101KPa. After the temperature reaches 101KPa, the specimens are kept in the furnace for 20min~25min, and then taken out of the furnace for air cooling. S3. Prepare the sample after heat treatment: the cross section is subjected to the steps of rough grinding, fine grinding, polishing and etching in sequence; S4. Place the eroded sample under an optical microscope to observe its structural changes, so as to distinguish whether the abnormal structure at the edge of the sample is a pseudo-eutectoid structure or a carburized structure.

2. The method for rapidly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel according to claim 1, characterized in that: In step S1, a grinding wheel cutter is used to prepare the sample, and the cutting fluid is kept to cool the sample at all times during the sample processing process.

3. The method for rapidly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel according to claim 1, characterized in that: In step S2, the air in the furnace is exhausted until the pressure gauge shows 80 KPa, which is a low vacuum state.

4. The method for rapidly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel according to claim 1, characterized in that: In step S2, the heat treatment temperature is 850° C. to 870° C., and the furnace is kept warm for 20 minutes to 25 minutes.

5. The method for rapidly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel according to claim 1, characterized in that: In step S2, the temperature is raised while adding inert protective atmosphere nitrogen until the pressure gauge shows 101 KPa.

6. The method for rapidly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel according to claim 1, characterized in that: In step S2, the sample is air-cooled after being taken out of the furnace, and natural cooling is used to ensure the precipitation of the ferrite structure of the sample.

7. The method for rapidly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel according to claim 1, characterized in that: In step S3, the heat-treated sample is coarsely ground by a spectral grinding machine, then finely ground by 180-mesh, 600-mesh, and 1000-mesh metallographic sandpaper in sequence, and finally polished. The entire process requires cooling with water to prevent the grinding surface of the sample from overheating.

8. The method for rapidly distinguishing pseudo-eutectoid or carburized structure on the surface of medium and low carbon steel according to claim 1, characterized in that: In step S3, the etchant is a 2% nitric acid alcohol solution, and the polished surface of the sample is immersed in the etchant for 19 seconds to 21 seconds. The sample is slightly shaken during the etch process to maintain etch uniformity.

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