Manufacturing method of artificial steel micro-crack

Abstract

The present application relates to the technical field of artificial steel micro-crack manufacturing, and particularly relates to a method for manufacturing artificial steel micro-cracks. In the conventional method for manufacturing artificial micro-cracks on the surface of steel, the obtained artificial micro-cracks are quite different from the real micro-cracks on the surface of steel products, and it is difficult to obtain the real causes of the micro-cracks on the surface of steel products by comparing the artificial micro-cracks with the real micro-cracks. In view of the above problems, the present application provides a method for manufacturing artificial steel micro-cracks. The columnar crystal zone of a continuous casting billet is cut for sampling, and then is subjected to fracturing and reverse pressing. The obtained artificial micro-cracks have an opening less than 100 microns and have finer internal cracks, and are more consistent with the real micro-crack morphology formed on the surface of steel. The present application provides a better research idea for the source and control scheme of the micro-cracks on the surface of steel products.

Description

Technical Field

[0001] This invention relates to the field of artificial steel microcrack fabrication technology, specifically to a method for fabricating artificial steel microcracks. Background Technology

[0002] The production process of steel products includes continuous casting, billet heating, rolling, and other stages. After the steel solidifies, surface cracks may appear on its surface due to various reasons (such as unsuitable mold flux, poor secondary cooling, excessive straightening force, inappropriate billet heating temperature, and rolling scratches). Steel products with surface cracks will not disappear in subsequent forging, extrusion, drawing, and other processing steps, and will greatly harm the surface quality, service life, and mechanical properties of the final steel product. Therefore, controlling surface cracks in steel products is essential.

[0003] To better control surface cracks in steel products, it is essential to first understand how surface cracks are generated and at what stage they begin. Currently, research on the origin of microcracks on steel surfaces typically relies on observing the morphology of microcracks based on production experience and roughly inferring the cause of microcrack formation based on the degree of decarburization around the microcracks. However, this method often leads to misjudgments and cannot quickly and accurately solve the problem.

[0004] To pinpoint the exact stage of steel product manufacturing where microcracks originate, more detailed analysis and judgment are required. This involves simulating microcracks at different production stages, casting artificially created crack samples under identical casting conditions, and then carefully comparing the resulting samples with the surface crack morphology of the final steel product. This allows for easy identification of the specific production stage at which the microcracks originated. This method significantly reduces misjudgments of the causes of microcracks and provides fundamental theoretical guidance for understanding the origin of microcracks on the surface of steel products.

[0005] Currently, the conventional method for artificially creating microcracks on steel surfaces involves using wire cutting machines or other cutting machines to create crack-like defects on the steel surface. While this method is simple, it has several drawbacks, such as a large crack opening (wire cutting typically produces cracks larger than 400 μm, while real cracks are usually smaller than 100 μm), excessively straight cracks, and a lack of further crack propagation, making it impossible to obtain finer internal cracks. The artificial microcracks obtained by these conventional methods differ significantly from real microcracks on the steel product surface, making it difficult to determine the true cause of the microcracks by comparing them with real microcracks on the steel product surface. Summary of the Invention

[0006] A problem with existing technologies is that conventional methods for artificially creating microcracks on steel surfaces produce artificial microcracks that differ significantly from real microcracks on the steel product surface, making it difficult to determine the true cause of the microcracks by comparing them with real microcracks. To address this problem, this invention provides a method for creating artificial microcracks in steel, comprising the following steps:

[0007] (1) Select steel continuous casting billets;

[0008] (2) Cut samples from the columnar crystal region of the steel continuous casting billet to obtain several rectangular columnar crystal sample strips.

[0009] (3) The columnar crystal sample strip has two surfaces perpendicular to the growth direction of the columnar crystal. A groove is cut into one of the two surfaces by line cutting. The surface with the groove is denoted as surface a, and the other surface is denoted as surface b (surface b is parallel to and opposite to surface a). The length of the groove is equal to the width of surface a, and the axis of the groove is parallel to the side length of the width direction of surface a.

[0010] (4) Place the columnar crystal sample strip horizontally on the experimental platform with surface a facing down and surface b facing up. Apply a vertically downward force to surface b to perform fracturing.

[0011] (5) The columnar crystal sample strip is rotated 90° along surface a, and a vertical downward force is applied to surface b again for reverse pressing.

[0012] Preferably, the continuous casting billet of the steel is a square billet or a rectangular billet. In a square or rectangular billet, the columnar crystals are located in relative horizontal and vertical positions. If it is a circular billet, the columnar crystals are distributed radially along the circle. When cutting long strips of columnar crystal samples, cracks are easily caused to propagate radially along the columnar crystals, and skewed cracks are easily obtained.

[0013] Preferably, the cross-sectional dimensions of the steel continuous casting billet are ≥100×100mm.

[0014] Preferably, the thickness, width, and length of the columnar crystal sample strip are 10-15 mm, 10-15 mm, and 100-150 mm, respectively. The dimensions of the columnar crystal sample strip facilitate subsequent fracturing experiments using a bending testing machine. When the thickness and width of the columnar crystal sample strip are <10 mm, crack propagation easily penetrates the sample, potentially burning it off during subsequent high-temperature heating, leading to test failure. When the thickness and width are >15 mm, there is significant sample waste, and the excessive size makes processing difficult. When the length of the columnar crystal sample strip is <100 mm, it cannot be placed at either end of the bending machine pad. When the length of the columnar crystal sample strip is >150 mm, there is significant sample waste, and the excessive size makes processing difficult. A regular rectangular shape for the columnar crystal sample strip allows for direct sample preparation and analysis of the obtained artificial microcracks perpendicular to the crack direction. If the columnar crystal sample strip is cylindrical or other irregularly shaped, it not only affects crack fabrication but also requires further cutting and processing for subsequent sample observation.

[0015] Preferably, the depth of the groove is 0.5-1 mm. The purpose of wire EDM grooving is to pre-create an artificial defect on surface a of the columnar crystal sample strip, facilitating crack propagation along the largest surface defect (i.e., the defect created by the wire EDM grooving) during bending. Controlling the grooving depth controls the crack propagation depth. When the grooving depth is <0.5 mm, crack propagation is hindered, potentially leading to the sample failing to crack immediately and bending, ultimately resulting in sample breakage. When the grooving depth is >1 mm, the resulting crack after fracturing and reverse pressing does not accurately reflect the morphology of actual microcracks. This makes it impossible to accurately simulate the changes in the micromorphology of surface cracks during subsequent processing, such as decarburization and oxidation, in subsequent experiments. Furthermore, excessively deep grooving can cause propagating cracks to penetrate the sample, leading to sample breakage. Therefore, the optimal depth for wire EDM grooving is 0.5 mm-1 mm.

[0016] Preferably, during fracturing, the vertically downward pressure is applied by the bending arm of the bending testing machine, with a pressure of 150-180 kN and a compression displacement of 0.2-0.5 mm. Considering the sample size and the machine's capacity, a bending testing machine pressure <150 kN will result in a compression displacement <0.2 mm, leading to an excessively small crack in the sample; while a pressure >180 kN can easily cause a compression displacement >0.5 mm, resulting in brittle fracture of the sample and making it impossible to control the crack depth. Therefore, the optimal pressure is 150-180 kN, with a compression displacement of 0.2-0.5 mm.

[0017] Preferably, during reverse pressing, the vertically downward pressure is applied by the bending arm of the bending testing machine, with a pressure of 200-250 kN and a pressing displacement of 0.3-0.6 mm. Reverse pressing is mainly used to tighten wide, expanded cracks and control the crack gap to match the actual crack width. When the pressure is <200 kN and the pressing displacement is <0.3 mm, the requirement of crack opening <100 μm cannot be guaranteed; when the pressure is >250 kN and the pressing displacement is >0.6 mm, the sample is easily pressed into a depression, and bulging deformation occurs on both sides of the sample. It is also easy to cause secondary crack propagation along the vertical direction of the main crack, resulting in a significant change in the overall crack morphology.

[0018] The present invention has the following beneficial effects:

[0019] The artificial microcracks obtained by the method of the present invention have an opening of less than 100 μm and extend with even finer internal cracks, which more closely resemble the morphology of real microcracks formed on the surface of steel. This provides a better research approach for the source and control of microcracks on the surface of steel products. Attached image description:

[0020] Figure 1 : Schematic diagram of the process for creating artificial microcracks in Embodiment 1 of the present invention.

[0021] Figure 2 Artificial microcracks obtained on the surface of steel in Example 1.

[0022] Figure 3 Artificial microcracks obtained on the surface of steel in Comparative Example 1.

[0023] Figure 4 Artificial microcracks obtained on the surface of steel in Comparative Example 2.

[0024] Figure 5 Artificial microcracks obtained on the surface of steel in Comparative Example 3.

[0025] Figure 6 Artificial microcracks obtained on the surface of steel in Comparative Example 4.

[0026] Figure 7 Artificial microcracks obtained on the surface of steel in Comparative Example 5.

[0027] Figure 8 Artificial microcracks obtained on the surface of steel in Comparative Example 6.

[0028] Figure 9 Artificial microcracks obtained on the surface of steel in Comparative Example 7.

[0029] Figure 10 Artificial microcracks obtained on the surface of steel in Comparative Example 8.

[0030] Figure 11Artificial microcracks obtained on the surface of steel in Comparative Example 9. Detailed implementation method:

[0031] The present invention will be described in detail below with reference to embodiments. However, it should be understood that the following embodiments are merely illustrative examples of implementation of the present invention and are not intended to limit the scope of the present invention.

[0032] Example 1

[0033] Taking 40Cr steel as an example, artificial microcracks are created. The specific method for creating artificial microcracks in steel is as follows:

[0034] (1) The cross-sectional dimensions of the 40Cr steel billet are 140×140mm. At a position 35mm from the surface of the billet (which is exactly the columnar crystal region, and the growth direction of the columnar crystals is as shown in the attached instruction manual). Figure 1 As shown, along the periphery of the billet cross-section pointing towards the inside of the cross-section (the growth direction of the columnar crystals is perpendicular to the axis of the billet's extension direction), a steel sheet with a thickness of d = 11 mm is cut downwards along a horizontal direction perpendicular to the end face of the billet. Then, the obtained steel sheet is divided into several columnar crystal sample strips along its width direction. Each columnar crystal sample strip is a cuboid with a thickness, width, and length of 11 mm, 11 mm, and 140 mm, respectively.

[0035] (2) Take a columnar crystal sample strip with two surfaces perpendicular to the growth direction of the columnar crystal. Make a groove by wire cutting on one of the surfaces. The surface with the groove is called surface a, and the other surface is called surface b. The length of the groove is equal to the width of surface a. The axis of the groove is parallel to the side length of the width direction of surface a. The depth of the groove is 0.6 mm.

[0036] (3) Place the columnar crystal sample strip obtained in step (2) horizontally on the experimental platform with side a facing down and side b facing up. Use a bending tester to apply a vertical downward force on side b to perform fracturing. The pressure is 160KN and the displacement is controlled to be 0.3mm.

[0037] (4) The columnar crystal sample strip is rotated 90° clockwise along surface a, and a bending tester is used to apply a vertical downward force on surface b for reverse pressing. The pressure is 220KN and the pressing displacement is controlled at 0.5mm.

[0038] Example 1: Microcracks obtained on the surface of steel, as shown in the attached instruction manual. Figure 2 As shown, the microcrack opening width is 56 μm, and it extends into even finer internal cracks.

[0039] Comparative Example 1 is the same as Example 1, except that in Comparative Example 1, a steel sheet with a thickness of d = 11 mm was cut downwards along a horizontal direction perpendicular to the end face of the steel billet at a position 70 mm from the surface of the billet. The microcracks on the surface of the steel obtained in Comparative Example 1 are as shown in the appendix to the specification. Figure 3 As shown in the image, the crack opening width of Comparative Example 1 is 63 μm, but the crack is a non-vertical oblique crack, which is not convenient for studying the characteristics at different depths of the crack and interferes with the subsequent high-temperature oxidation treatment.

[0040] Comparative Example 2 is the same as Example 1, except that the columnar crystal sample in Comparative Example 2 did not have grooves on its surface. To induce cracks in the sample, a pressure of 300 kN and a specific displacement were required. The surface morphology of the steel obtained in Comparative Example 2 is shown in the attached specification. Figure 4 As shown in the image, the crack opening width is 68 μm. The crack is also a vertical crack, but it runs through the entire sample. Subsequent tests will have the problem of simultaneous high-temperature oxidation at both ends, which will not only interfere with the test results, but also make the sample very easy to break.

[0041] Comparative Example 3 is the same as Example 1, except that the depth of the groove on surface a of the columnar crystal sample in Comparative Example 3 is 5 mm. The microcracks on the surface of the steel obtained in Comparative Example 3 are as shown in the appendix to the specification. Figure 5 As shown in the image, the crack opening width is 59 μm, and the crack is also a vertical crack. However, the crack penetrates the entire sample, which poses a problem of simultaneous high-temperature oxidation at both ends in subsequent tests. This not only interferes with the test results, but also makes the sample extremely prone to fracture.

[0042] Comparative Example 5 is the same as Example 1, except that the pressure in step (3) of Comparative Example 5 is 100 kN and the displacement is 0.1 mm. The microcracks on the surface of the steel obtained in Comparative Example 5 are as shown in the attached instruction manual. Figure 6 As shown in the image, the crack opening width is 70 μm, and the crack is vertical. However, the crack depth is too shallow, making it difficult to study the morphological changes at different crack depths along the entire crack propagation direction.

[0043] Comparative Example 6 is the same as Example 1, except that in step (3) of Comparative Example 6, the pressure is 200 kN and the displacement is 0.7 mm. The microcracks on the surface of the steel obtained in Comparative Example 6 are as shown in the attached instruction manual. Figure 7 As shown in the image, the crack opening width is 66 μm, and the crack is vertical. However, the crack penetrates the entire sample, which poses a problem of simultaneous high-temperature oxidation at both ends in subsequent tests. This not only interferes with the test results but also makes the sample extremely prone to fracture.

[0044] Comparative Example 7 is the same as Example 1, except that in step (4) of Comparative Example 7, the pressure is 160 kN and the displacement is 0.25 mm. The microcracks on the surface of the steel obtained in Comparative Example 7 are as shown in the attached instruction manual. Figure 8 As shown in the image, the crack opening width is 231 μm, which does not conform to the typical characteristic of an actual crack ≤100 μm. In subsequent high-temperature oxidation tests, the crack opening is likely to be overly oxidized due to its excessive size, making it impossible to compare with the actual situation.

[0045] Comparative Example 8 is the same as Example 1, except that in step (4) of Comparative Example 8, the pressure is 300 kN and the displacement is 0.8 mm. The microcracks on the surface of the steel obtained in Comparative Example 8 are as shown in the attached instruction manual. Figure 9 As shown in the image, the crack opening width is 89 μm, but there are branch cracks near the main trunk of the vertical crack, which makes it difficult to analyze the crack morphology characteristics during subsequent high-temperature oxidation tests using a single-factor method.

[0046] Comparative Example 9 is the same as Example 1, except that the reverse pressing in step (4) was not performed in Comparative Example 9. The microcracks on the surface of the steel obtained in Comparative Example 9 are as shown in the appendix to the specification. Figure 10 As shown in the image, the crack opening width is 731 μm, which does not conform to the typical characteristic of an actual crack ≤100 μm. In the subsequent high-temperature oxidation test, the crack opening will be too large and the oxidation will be too severe, making it impossible to compare with the actual situation. Therefore, the results of artificially pre-fabricated cracks are not good.

[0047] Comparative Example 10

[0048] (1) The billet of 40Cr steel has a cross-sectional size of 140×140mm. At a position 35mm away from the surface of the billet, a steel sheet with a thickness of d=11mm is cut downward along the horizontal direction perpendicular to the end face of the billet. Then, the obtained steel sheet is divided into several columnar crystal sample strips along its width direction. Each columnar crystal sample strip is a cuboid with a thickness, width and length of 11mm, 11mm and 140mm respectively.

[0049] (2) Take a columnar crystal sample strip with two faces perpendicular to the growth direction of the columnar crystals. Make a groove by wire cutting on one of these faces. The face with the groove is denoted as face a, and the other face as face b. The length of the groove is equal to the width of face a, and the axis of the groove is parallel to the side length of face a in the width direction. The depth of the groove is 8 mm. The microcracks on the surface of the steel obtained in Comparative Example 10 are as shown in the attached instruction manual. Figure 11 As shown in the image, the aperture is 436 μm, which does not possess the typical characteristics of a crack.

[0050] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for creating artificial microcracks in steel, characterized in that, Includes the following steps: (1) Select steel continuous casting billets; (2) Cut samples from the columnar crystal region of the steel continuous casting billet to obtain several rectangular columnar crystal sample strips; (3) The columnar crystal sample strip has two surfaces perpendicular to the columnar crystal growth direction. A groove is made on one of the surfaces, which is denoted as surface a. The other surface is denoted as surface b. The length of the groove is equal to the width of surface a, and the axis of the groove is parallel to the side length of the width direction of surface a. (4) Place the columnar crystal sample strip horizontally on the experimental platform with surface a facing down and surface b facing up. Apply a vertically downward force to surface b to perform fracturing. (5) The columnar crystal sample strip is rotated 90° along surface a, and a vertical downward force is applied to surface b again for reverse pressing; The depth of the trench is 0.5-1 mm; During fracturing, pressure is applied vertically downwards to surface b, with a pressure of 150-180 kN and a displacement of 0.2-0.5 mm. During reverse pressing, pressure is applied vertically downwards to surface b, with a pressure of 200-250KN and a pressing displacement of 0.3-0.6mm.

2. The method for fabricating artificial steel microcracks according to claim 1, characterized in that, The continuous casting billet of steel is a square billet or a rectangular billet.

3. The method for fabricating artificial steel microcracks according to claim 1, characterized in that, The cross-sectional dimensions of the continuously cast steel billet are ≥100×100mm.

4. The method for fabricating artificial steel microcracks according to claim 1, characterized in that, The thickness, width, and length of the columnar crystal sample strips are 10-15 mm, 10-15 mm, and 100-150 mm, respectively.

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