Detection method for second-phase banded structure of medium-low carbon steel

Abstract

The invention discloses a method for detecting a second-phase banded structure of medium-low carbon steel, which comprises the following steps of: (1) preparing and corroding a sample, and then placing the sample under a microscope to measure the grain size; (2) determining the detected view field size according to the grain size level; (3) classifying the second phase distribution state in the detected view field size range; and (4) representing the non-uniform degree of the second phase structure distribution according to the distribution pattern of the segregation band of the second phase structure. The detection method is simple and feasible, and can be implemented in work.

Description

Technical Field

[0001] The invention relates to a detection method for measuring the second phase tissue band in the microstructure of medium and low carbon steel, and is applied in the field of steel material detection. Background Art

[0002] During the forging or rolling process, steel plates are heated above the austenitizing temperature. During the subsequent cooling process, austenite transforms into ferrite, pearlite, bainite, and martensite, forming microstructures composed of different phases at room temperature. In the same sample, ferrite has a low carbon content and low hardness, while bainite, martensite, and pearlite have high carbon contents and high hardness. Under a microscope, segregation is primarily manifested in the distribution of phases, with the most prevalent phases being the primary phases. The primary phase generally refers to the phase that constitutes the basic structure of the material and generally has a continuous spatial distribution, making it the primary phase. The secondary phase refers to the microstructure of the material other than the primary phase.

[0003] The second phase band is formed by the aggregation of phases other than the base phase, including ferrite, pearlite, bainite, and other non-equilibrium structures, either singly or mixed together. This band is formed when the temperature of the molten steel drops during solidification, and alloying elements precipitate, causing the later solidified part to form a different phase and composition from the earlier solidified part. The aggregation of similar phases forms this second phase band, which results in compositional differences. Segregation can lead to an uneven microstructure within the material. Under the action of external forces, the deformation is inconsistent, and cracks form between the two phases, reducing the overall strength and toughness of the material. For example, if the center of a steel plate has severe segregation, a delamination fracture will appear during a tensile test, seriously affecting the strength and ductility of the steel plate. Therefore, the detection of segregation is an important means to understand the uniformity of the material and is necessary.

[0004] Currently, low-magnification acid immersion etching is commonly used to detect material segregation. Segregation detection targets include ingot-shaped segregation, spot segregation, and center segregation. This method primarily targets ingots, which are considered macrosegregation and are a key component of continuous casting process control. However, the product, when delivered or used, is in the form of steel plates. This segregation distribution is the result of heating, forging, rolling, and cooling of the ingot, making low-magnification acid immersion etching unsuitable.

[0005] In the testing of low- and medium-carbon steels, the examination of banded structure can, to a certain extent, reflect the degree of segregation in forged or rolled products. For example, the standard banded structure rating chart in GB34474.1 grades products based on the ferrite bands formed during microstructural transformation. This rating method reflects the microscopic segregation of products produced under relatively slow cooling rates. However, with the current trend toward microalloying and rapid cooling in steel production processes, the ferrite bands formed are discontinuous, while other, harder second-phase bands are relatively clear. The GB34474.1 method for these second-phase bands formed under rapid cooling cannot accurately reflect the degree of microstructural segregation. Furthermore, because this testing method is performed at a 100x field of view, the results are significantly affected by grain size. Smaller grains make it more difficult for the bands to penetrate the field of view, resulting in a low rating and difficulty in achieving uniformity in banding morphology and grade. Furthermore, current testing methods fail to quantitatively measure the uneven distribution of second-phase structures in a material.

[0006] Therefore, a new detection method is needed to detect the second phase tissue band, make the detection results uniform, and achieve the purpose of quantitative measurement. Summary of the Invention

[0007] To solve the problems of the prior art, the present invention provides a method for detecting the second phase banded structure of medium and low carbon steel. This method combines traditional detection methods with quantitative measurement to reflect the degree of material segregation.

[0008] A method for detecting the second phase banded structure of medium and low carbon steel comprises the following steps:

[0009] (1) After sample preparation and corrosion, observe under a microscope and measure the grain size;

[0010] (2) Determine the field size of the inspection based on the grain size level;

[0011] (3) Classify the distribution state of the second phase within the detection field size;

[0012] (4) Characterize the uneven distribution of the second phase structure based on the distribution morphology of the second phase structure segregation band.

[0013] Furthermore, the carbon content of the medium-low carbon steel is 0.0218-0.6wt%.

[0014] Furthermore, in step (1), the detection surface of the sample is mainly the longitudinal surface of the sample, and the location where the second phase tissue band is most severely and obviously distributed is selected for detection.

[0015] Furthermore, in step (1), the sample preparation and corrosion operations are as follows: firstly, the sample is mounted, then the test surface is ground and polished with sandpaper, and then the sample is corroded with a corrosion reagent and then blown dry.

[0016] Furthermore, in step (1), the detection object is mainly the second phase tissue band in the sample microstructure, wherein the second phase tissue band is an aggregated tissue composed of other phases excluding the basic phase, and can be a single phase or a mixed phase.

[0017] Furthermore, in step (1), the etching reagent is a nital solution, an aqueous solution of ferric chloride and hydrochloric acid, or the like, which can clearly display the tissue morphology. The concentration of the nital solution is 2-5%, and the ratio of ferric chloride, hydrochloric acid, and water in the aqueous solution of ferric chloride and hydrochloric acid is 1-2 g: 1-2 ml: 10 ml.

[0018] Furthermore, in step (1), in order to ensure that the collected image can meet the detection requirements, it is necessary to select an appropriate magnification to observe and collect images of the second phase tissue band rating area. The method for selecting the magnification is as follows: when the grain size level G is less than level 6, the magnification of the microscope is ≤25X; when the grain size level G is: 8>G≥6, the magnification of the microscope is 50X; when the grain size level G is: 10>G≥8, the magnification of the microscope is 100X; when the grain size level G is: 12>G≥10, the magnification of the microscope is 200X; when the grain size level G is: 14>G≥12, the magnification of the microscope is 400X; when the grain size level G is ≥14, the magnification of the microscope is ≥800X.

[0019] Furthermore, in step (2), the shape of the visual field used for evaluation is a square.

[0020] Furthermore, in step (2), in order to prevent the failure to capture a sufficiently large detection field at high magnification, the standard image side length at 100X is 710um and the area is 0.5mm, with the grain size of level 8. 2 The square selection area is used as the standard. At this time, the side length of the field of view is 35.5 grains, and the total length of 35.5 grains is used as the side length of the reference field of view.

[0021] Furthermore, in step (2), the formula for determining the side length E of the field of view to be detected according to the grain size is:

[0022] If the grain length is used as the reference size of the rating field, the size of the rating field will vary depending on the grain size of different samples. The functional relationship between the field size and the grain size is as follows:

[0023]

[0024] Among them, L is the average intercept of the grains, G is the grain size grade, and E is the side length of the rating field.

[0025] For example, if the grain size of the sample is 11.4, the width of the measurement field is 201μm. Under this rule setting, the relationship between the size of the rated field and the grain size can be fixed through a functional relationship.

[0026] Furthermore, in step (3), the second phase structure is classified according to the distribution degree of the second phase structure segregation band within the detection field size range, and the detection field is the most serious field.

[0027] The classification results are as follows:

[0028] Class I: All tissues are evenly distributed in the measurement field, and no second phase segregation band is observed;

[0029] Category II: The second phase tissue bands in the measurement field are discontinuously distributed and do not cross the field of view;

[0030] Category III: The second phase structure is continuously distributed within the measurement field of view and passes through the field of view.

[0031] Furthermore, in step (3), a square field of view of corresponding size is selected from the area with the most severe band distribution under the microscope to collect images.

[0032] Furthermore, in step (4), the area is measured and M is calculated using the following formula to characterize the uneven distribution degree (segregation degree) of the second phase structure;

[0033]

[0034] in, A is the average value of the measured second phase area; 总面积 is the total area of ​​the measured image; M is a reference value for measuring the unevenness of the size distribution of the second phase.

[0035] Under the same category of banded tissue, The smaller it is, the smaller the area of ​​a single second phase is or the more uniform its distribution is; The larger it is, the larger or more aggregated the single second phase area is.

[0036] Furthermore, in step (4), before calculation, the second phase in the captured image is subjected to image measurement software, and the contrast is adjusted, and manual editing is performed to enhance the color difference of the second phase tissue in the image, and the second phase is selected by binarization, and the area value is measured.

[0037] Furthermore, in step (4), During the calculation, we ignore any second phase smaller than the area of ​​one grain, and select those with an area larger than one grain for inclusion. Second phases smaller than the area of ​​one grain represent a normal distribution and have little impact on segregation. This method can also be used to filter out interfering factors such as oxide sites and grain boundaries formed by corrosion.

[0038] Furthermore, in step (4), the distribution uniformity of the second phase structure is calculated according to the formula, and the calculated value can be used to compare the segregation degree of the second phase in the same distribution state.

[0039] Beneficial effects of the present invention:

[0040] (1) The detection of the second phase tissue band can meet the requirements of the detection of the microsegregation of most materials and the uniformity of the second phase area distribution.

[0041] (2) This detection method can be implemented in work.

[0042] (3) This method associates the detection field of view with the grain size, so that the detection results are not affected by the grain size and are unified.

[0043] (4) This method can take into account the distribution state of the banded structure and can compare the segregation degree of the same type of second phase through the measured values. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is the pearlite band distribution diagram of the PHS1500 hot-rolled sample in Example 1.

[0045] Figure 2 This is the pearlite band distribution diagram of Q275 hot-rolled plate in Example 2.

[0046] Figure 3 This is the martensite band distribution diagram of DP780 cold-rolled dual-phase steel in Example 3.

[0047] Figure 4 This is the band distribution diagram of SPHC hot-rolled plate in Example 4. DETAILED DESCRIPTION

[0048] The following non-limiting examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0049] Example 1

[0050] A method for detecting the second phase banded structure of medium and low carbon steel, taking PHS1500 hot-rolled steel plate sample as an example, the steps are as follows:

[0051] 1) Sample preparation

[0052] The specimen was cut and inlaid on the longitudinal surface. The test surface was ground with 180#, 320#, 500# and 800# sandpaper in sequence. The test surface was polished with a polishing cloth and a polishing agent with a particle size of 3.0μm until the surface was smooth and free of scratches.

[0053] 2) Corrosion of the sample

[0054] For the configuration of chemical reagents, the corrosion reagent is selected as 4% nitric acid alcohol solution, and the corrosion is carried out for 2 seconds. The sample is taken out, rinsed with water and then blown dry.

[0055] 3) Sample testing

[0056] 3.1 The sample was placed under a microscope for observation at a magnification of 200X. The grain size was measured to be 11.4, with an average grain intercept of 6.2 μm. The side length of the inspection field was calculated to be 219 μm by the formula.

[0057] 3.2 Observing the structure under a microscope, the basic phase is ferrite and the secondary phase is pearlite, which appears as a band across the field of view and is Type III.

[0058] 3.3 The area of ​​the second phase was measured and M was calculated to be 1.08%.

[0059] Example 2

[0060] A method for detecting the second phase banded structure of medium and low carbon steel, taking Q275 hot-rolled steel plate sample as an example, the steps are as follows:

[0061] 1) Sample preparation

[0062] The specimen was cut and inlaid on the longitudinal surface. The test surface was ground with 180#, 320#, 500# and 800# sandpaper in sequence. The test surface was polished with a polishing cloth and a polishing agent with a particle size of 3.0μm until the surface was smooth and free of scratches.

[0063] 2) Corrosion of the sample

[0064] For the configuration of chemical reagents, the corrosion reagent is selected as 4% nitric acid alcohol solution, and the corrosion is carried out for 2 seconds. The sample is taken out, rinsed with water and then blown dry.

[0065] 3) Sample testing

[0066] 3.1 Place the sample under a microscope for observation at a magnification of 400X. Measure the grain size, which is level 12. The average grain intercept is 5 μm. The side length of the detection field is calculated to be 177.5 μm by the formula.

[0067] 3.2 Observing the structure under a microscope, the basic phase is ferrite and the second phase is pearlite. The second phase is discontinuously distributed and is Class II.

[0068] 3.3 The area of ​​the second phase was measured and M was calculated to be 0.19%.

[0069] Example 3

[0070] A method for detecting the second phase banded structure of medium and low carbon steel, taking DP980 cold-rolled dual-phase steel sample as an example, the steps are as follows:

[0071] 1) Sample preparation

[0072] The specimen was cut and inlaid on the longitudinal surface. The test surface was ground with 180#, 320#, 500# and 800# sandpaper in sequence. The test surface was polished with a polishing cloth and a polishing agent with a particle size of 3.0μm until the surface was smooth and free of scratches.

[0073] 2) Corrosion of the sample

[0074] For the configuration of chemical reagents, the corrosion reagent is selected as 4% nitric acid alcohol solution, and the corrosion is carried out for 2 seconds. The sample is taken out, rinsed with water and then blown dry.

[0075] 3) Sample testing

[0076] 3.1 Place the sample under a microscope for observation at a magnification of 400X. Measure the grain size, which is 13.5. The average grain intercept is 3 μm. The side length of the detection field is calculated to be 100.5 μm by the formula.

[0077] 3.2 The microstructure is observed under a microscope. The basic phase is ferrite and the secondary phase is martensite, which are evenly distributed and belong to Type I.

[0078] 3.3 The area of ​​the second phase was measured and M was calculated to be 0.06%.

[0079] Example 4

[0080] A method for detecting the second phase banded structure of medium and low carbon steel, taking SPHC hot-rolled plate specimens as an example, the steps are as follows:

[0081] 1) Sample preparation

[0082] The specimen was cut and inlaid on the longitudinal surface. The test surface was ground with 180#, 320#, 500# and 800# sandpaper in sequence. The test surface was polished with a polishing cloth and a polishing agent with a particle size of 3.0μm until the surface was smooth and free of scratches.

[0083] 2) Corrosion of the sample

[0084] For the configuration of chemical reagents, select 4% nitric acid alcohol solution as the corrosion reagent, corrode for 2 seconds, take out the sample, rinse with water and blow dry.

[0085] 3) Sample testing

[0086] 3.1 The sample was placed under a microscope for observation at a magnification of 500X. The grain size was measured to be level 14, with an average grain intercept of 2.5 μm. The side length of the detection field of view was calculated to be 88.7 μm by the formula.

[0087] 3.2 When observing the structure under a microscope, the basic phase is ferrite and the secondary phase is pearlite. The secondary phase structure is continuously distributed across the field of view and is Class III.

[0088] 3.3 The area of ​​the second phase was measured and M was calculated to be 0.4%.

Claims

1. A method for detecting the second phase banded structure of medium and low carbon steel, characterized in that: The steps include: (1) After sample preparation and corrosion, observe under a microscope and measure the grain size; (2) Determine the field size of the inspection based on the grain size level; (3) Classify the second phase distribution state within the detection field size; (4) The degree of uneven distribution of the second phase structure is characterized based on the distribution morphology of the second phase structure segregation band.

2. The detection method according to claim 1, wherein The carbon content of the medium-low carbon steel is 0.0218-0.6 wt %.

3. The detection method according to claim 1, wherein In step (1), the detection method is associated with the measurement of grain size, and the size of the detection field of view is determined by the grain size.

4. The detection method according to claim 1, wherein In step (1), the detection surface of the sample is the longitudinal surface of the sample, and the location where the second phase tissue band is most severely distributed and most obvious is selected for detection.

5. The detection method according to claim 1, wherein In step (1), the corrosion reagent used in the corrosion is nital alcohol solution or ferric chloride hydrochloric acid aqueous solution; wherein the concentration of the nital alcohol solution is 2-5%, and the ratio of ferric chloride, hydrochloric acid and water in the ferric chloride hydrochloric acid aqueous solution is 1-2g:1-2ml:10ml.

6. The detection method according to claim 1, characterized in that In step (1), a suitable magnification is selected to observe and capture images of the second phase tissue band; the method for selecting the magnification is as follows: when the grain size level G is less than level 6, the magnification of the microscope is ≤25X; when the grain size level G is: 8>G≥6, the magnification of the microscope is 50X; when the grain size level G is: 10>G≥8, the magnification of the microscope is 100X; when the grain size level G is: 12>G≥10, the magnification of the microscope is 200X; when the grain size level G is: 14>G≥12, the magnification of the microscope is 400X; when the grain size level G is ≥14, the magnification of the microscope is ≥800X.

7. The detection method according to claim 1, characterized in that In step (1), the second phase tissue band is an aggregated tissue composed of other phases excluding the basic phase, and is a single phase or a mixed phase.

8. The detection method according to claim 1, wherein In step (2), the shape of the evaluation field is a square, and the formula for determining the side length E of the field of view to be tested according to the grain size is: Among them, L is the average intercept of the grains, and G is the grain size grade.

9. The detection method according to claim 1, wherein In step (3), the area is measured and M is calculated using the following formula to characterize the uneven distribution of the second phase tissue; in, A is the average value of the measured second phase area; 总面积 is the total area of ​​the image, and M is a reference value for measuring the unevenness of the size distribution of the second phase.

10. The detection method according to claim 1, characterized in that In step (4), during the calculation of A, a second phase having an area greater than that of one grain is selected.

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