Decarburized layer thickness detection method based on coercive force detection

Abstract

The invention relates to a coercive force-based decarburized layer thickness detection method, which relates to the field of decarburized layer thickness nondestructive detection, and comprises the following steps: (1) establishing a mathematical model between coercive force and decarburized layer thickness; and (2) determining the thickness of the decarburized layer by detecting the coercive force. The method for determining the thickness of the decarburized layer by detecting the coercive force comprises the following steps: measuring a hysteresis loop of a metal surface to be measured, and calculating the coercive force; and the coercive force is compared with a mathematical model between the coercive force of the corresponding metal and the thickness of the decarburized layer, and the thickness of the decarburized layer is determined. Compared with a traditional metallographic method, a hardness method, a carbon content determination method and other decarburized layer thickness detection methods, the method adopts a nondestructive detection technology based on coercive force. In this way, in the detection process, destructive sampling or treatment does not need to be conducted on the metal plate, so that the integrity of the material is protected, and material waste caused by detection is avoided.

Description

Technical Field

[0001] The invention relates to a decarburized layer thickness detection method, and in particular to plate coercivity detection, decarburized layer thickness calibration, and coercivity and decarburized layer thickness mathematical model construction. Background Art

[0002] The decarburized layer is a thin layer formed when the carbon content on the surface of the metal sheet decreases due to high temperature during processing. Its thickness has an important influence on the mechanical properties, corrosion resistance and subsequent processing performance of the material. By detecting the thickness of the decarburized layer, the decarburization problem on the surface of the material can be discovered in time, providing a reliable basis for material quality control and performance evaluation. At the same time, this also helps to optimize the production process, reduce material waste and increased production costs caused by decarburization, and improve the competitiveness and market value of metal products. Therefore, the detection of the thickness of the decarburized layer on the surface of the plate is an indispensable part of the production and processing of metal materials.

[0003] Commonly used methods for detecting the thickness of the decarburized layer include metallographic method, hardness method, carbon content determination method, etc., but they all have various problems such as being greatly affected by human factors and damaging the measured surface. The coercive force detection is used to determine the thickness of the decarburized layer on the surface of the plate, that is, by exciting a magnetic field on the surface of the metal plate, detecting the electromagnetic response characteristics of its surface, calculating the coercive force, and quantifying the correlation between the coercive force and the thickness of the decarburized layer through numerical fitting, constructing a mathematical model between the coercive force and the thickness of the decarburized layer, and then realizing non-destructive, convenient and efficient detection of the thickness of the decarburized layer, which can effectively overcome the shortcomings of other detection methods and has high application value.

[0004] After searching, patent 202011012747.0 discloses a method for detecting the thickness of rail decarburization layer based on electromagnetic response. This detection method is a method for testing the thickness of rail decarburization layer based on the principle of electromagnetic response. Its implementation method relies on the zero-crossing frequency of electromagnetic induction, which is not closely related to the change of carbon content of the material. Therefore, the problem is that the stability and reliability are insufficient. What we rely on is coercive force, which is more closely related to the change of carbon content of the material. Among the various influencing factors of magnetic materials, coercive force has the advantages of less interference from external influences and less discrete measurement results when detecting the thickness of the decarburization layer. Summary of the invention

[0005] The present invention aims to provide a decarburized layer thickness detection method based on coercive force detection, which is simple to operate, low in cost, has few influencing factors, and can improve the efficiency and quality of decarburized layer thickness detection.

[0006] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme, including:

[0007] Decarburized layers are prepared on metal sheets, and decarburized layers of different thicknesses are produced on the surfaces of different samples by heating, heat preservation and cooling. The thickness of the decarburized layers on the surfaces of different samples is observed by metallographic method. The hysteresis loop of the sample surface is measured by a coercive force measuring device composed of a power supply, a signal generating module, a detection probe, a signal acquisition module and a signal processing module, and the coercive force is calculated. The thickness of the decarburized layers on the surfaces of different samples is matched one by one with the corresponding coercive force values, and the two are numerically fitted to establish a mathematical model between the coercive force and the thickness of the decarburized layers.

[0008] 1) The preparation of the decarburized layer on the metal sheet is achieved by heating, heat preservation and cooling, wherein the heating rate, peak temperature, heat preservation time, cooling method and time are related to the sheet material. These parameters directly determine whether a uniform decarburized layer can be prepared, and also determine the thickness of the prepared decarburized layer. Therefore, during the preparation, it is necessary to reasonably determine the above parameters according to the type of sheet material to ensure that a uniform decarburized layer with different thicknesses can be prepared on different sheet samples.

[0009] 2) The coercive force measuring device is composed of a power supply, a signal generating module, a detection probe, a signal acquisition module, a signal processing module, and an oscilloscope. The signal generating module excites a sinusoidal current signal, which is amplified by the signal processing module and applied to the surface of the measured plate through the detection probe, so that the surface of the measured plate generates an electromagnetic response. The detection probe collects the values ​​of magnetic induction intensity and magnetic field intensity, and after being processed by the signal processing module, the hysteresis loop image is displayed on the oscilloscope.

[0010] 3) The coercive force is calculated based on the hysteresis loop image, taking the horizontal coordinates of the two intersections of the image and the X-axis, and then calculating the coercive force value through the formula, as shown in the following formula:

[0011] Hc=(|x 1 |+|x 2 |) / 2

[0012] Where Hc represents the coercive force, unit is A / m; x 1 、x 2 It is the magnetic field intensity corresponding to the intersection of the hysteresis loop and the X-axis, in A / m.

[0013] 4) The metallographic method is used to observe the thickness of the decarburized layer on the surface of different samples. The steps of sampling, cold mounting, rough grinding, fine grinding, metallographic sample polishing, chemical etching, etc. are carried out in sequence, and then the thickness of the decarburized layer is observed through a metallographic microscope. If there is a serious uneven decarburized layer on the surface of the plate sample, it is necessary to sample at multiple points and take the average value of the decarburized layer thickness at each measuring point as the decarburized layer thickness value of the sample surface.

[0014] 5) The decarburized layer thickness on the surface of different samples is matched to the corresponding coercive force value. In order to obtain the numerical fitting sample, it is necessary to prepare multiple decarburized layers of different thicknesses for multiple plate samples, and control the decarburized layer thickness by changing the preparation parameter of "heat preservation time" when the plate material is the same; the coercive force value of each sample surface is tested by using a coercive force measuring device, and the coercive force value is mapped to the decarburized layer thickness one by one.

[0015] 6) The mathematical model between the coercive force and the thickness of the decarburized layer is established. Taking the mapping relationship between the coercive force value and the thickness of the decarburized layer as a sample, the mathematical law between the coercive force and the thickness of the decarburized layer is found by using the numerical fitting method, and a quantitative model between the coercive force and the thickness of the decarburized layer is established, so as to achieve the goal of measuring the thickness of the decarburized layer by coercive force detection.

[0016] When detecting the thickness of the decarburized layer of the metal material, it is only necessary to measure the hysteresis loop of the metal surface to be tested and calculate the coercive force; the coercive force is compared with the mathematical model between the coercive force of the corresponding metal and the decarburized layer thickness to determine the decarburized layer thickness.

[0017] Beneficial effects:

[0018] 1. Compared with the traditional metallographic method, hardness method, carbon content determination method and other decarburized layer thickness detection methods, the present invention adopts non-destructive detection technology based on coercivity. This means that during the detection process, there is no need to destructively sample or process the metal sheet, thereby protecting the integrity of the material and avoiding material waste caused by detection.

[0019] 2. Compared with the technical means that rely on electromagnetic induction zero-crossing frequency to test the thickness of the decarburized layer of metal materials, the present invention has obvious advantages. This solution relies on the detection of coercive force. The coercive force is not obtained through the zero-crossing frequency. It is calculated based on the hysteresis loop. The shape of the hysteresis loop (that is, the size of the coercive force) will be affected by the size of the grain size of the material. The grain size of the decarburized layer of the material is not consistent with that of the base layer, and the change in grain size is very sensitive to the hysteresis loop (coercive force). The measurement accuracy is higher, that is, among the various influencing factors of magnetic materials, the coercive force has the advantages of less interference from external influences and small discreteness of measurement results when detecting the thickness of the decarburized layer. Patent 202011012747.0 relies on the relationship between the zero-crossing frequency and the thickness of the decarburized layer for measurement. It is mainly obtained through multi-frequency scanning. The zero-crossing frequency is not close enough to the change in the carbon content of the material. The accuracy of the zero-crossing frequency is greatly affected by the scanning frequency and the severity of the change in the organizational structure. Therefore, this solution has higher accuracy and reliability.

[0020] 3. The present invention realizes the rapid prediction of the thickness of the decarburized layer by constructing a mathematical model between the coercive force and the thickness of the decarburized layer. The thickness of the decarburized layer can be directly obtained through the mathematical model by simply measuring the hysteresis loop of the metal surface to be tested and calculating the coercive force, without the need for cumbersome subsequent processing and analysis steps, which greatly improves the detection efficiency and convenience.

[0021] 4. The method of the present invention is applicable to the detection of the decarburized layer thickness of various metal plates. It only needs to reasonably determine the sample preparation parameters such as heating, heat preservation, and cooling according to the type of plate material to prepare samples with different decarburized layer thicknesses and build corresponding mathematical models. Therefore, the method has strong adaptability and flexibility and can meet the requirements of decarburized layer thickness detection under different materials and process conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the work flow chart of the present invention.

[0023] Figure 2 This is a physical picture of the sample prepared when developing the mathematical model of this embodiment.

[0024] Figure 3 This is a fitting diagram of the coercive force value and the decarburized layer thickness of this embodiment.

[0025] Figure 4 This is a diagram of the sample to be tested with points to be tested marked in this embodiment.

[0026] Figure 5 This is a sample picture after cutting in this embodiment.

[0027] Figure 6 This is a metallographic photograph of test point No. 1 of sample in this embodiment.

[0028] Figure 7 This is a metallographic photograph of test point No. 2 of sample in this embodiment.

[0029] Figure 8 Metallographic photograph of test point No. 3 of the sample in this example.

[0030] Fig. 9 Metallographic photograph of test point No. 4 of the sample in this embodiment.

[0031] Fig.10 Metallographic photograph of test point No. 5 of the sample in this example. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of the present application. The terms used herein in the specification of the present application are only for the purpose of describing specific embodiments, not for limiting the present application.

[0033] See also Figure 1 , is a work flow chart of the present invention, comprising the following steps:

[0034] 1) Determine the test parameters. For the material type of the metal plate to be tested, the test determines the sample preparation test parameters such as heating rate, peak temperature, holding time, cooling rate, etc. that can produce a clear and relatively uniform decarburized layer. The metal plate tested in this embodiment is Q235b steel under the GB / T 700-2006 standard.

[0035] 2) Prepare samples. By selecting different test parameters such as holding time and cooling rate, samples with different decarburized layer thickness are prepared; Figure 2 As shown, the sample preparation method is: a plate sample with a size of 10*50*250mm is placed in a resistance furnace and heated to 800℃-810℃, and the temperature is maintained. The holding time is controlled to be 4, 5, 6, 7, 8, and 10h in sequence, and the sample is taken out and air-cooled after the specified time. After it is completely cooled, the surface oxide scale is removed by a relatively gentle means.

[0036] 3) Determine the area to be tested. Randomly select the area to be tested on the surface of the sample, and evenly select multiple points to be tested in the area to be tested;

[0037] 4) Perform a coercive force test on the area to be tested. The coercive force value of the area to be tested is measured using a coercive force testing device. Multiple tests should be performed at different locations in the area, and the average value is taken as the coercive force value of the area to be tested.

[0038] 5) Calibrate the thickness of the decarburized layer by metallographic method. In the area to be tested, the sample is cut to prepare the metallographic sample, and after cold mounting, rough grinding, fine grinding, metallographic sample polishing, chemical etching and other processes, the thickness of the decarburized layer in the area to be tested and at each test point is calibrated with a metallographic microscope, and the average value is taken as the thickness value of the decarburized layer in the area to be tested;

[0039] 6) Construct a mathematical model. Correspond the coercivity and decarburized layer thickness of the same area to be measured, establish a one-to-one mapping relationship between the coercivity and the decarburized layer thickness, and use linear regression fitting to obtain the mathematical model of the two;

[0040] Model verification and correction. Repeat steps 2-5, verify the model accuracy multiple times, and correct the mathematical model until the model accuracy meets the requirements. After verification and correction, the fitting diagram of the coercivity value and the decarburized layer thickness of the material is shown in Figure 3 , where the abscissa is the thickness of the decarburized layer in mm, and the ordinate is the coercive force value in A / M; according to the fitting diagram, the mathematical model of the coercive force value and the thickness of the decarburized layer of the material can be obtained as follows:

[0041] y=-61.994x 3 +68.331x 2 -24.99x+6.1696

[0042] R 2 =0.9606

[0043] In the formula, y represents the coercive force, the unit is A / m, and x is the thickness of the decarburized layer, the unit is mm. 2 It is called the coefficient of determination or goodness of fit, which indicates the strength of the correlation between the model prediction and the actual observed value. The closer it is to 1, the stronger the correlation, and the more accurate the fitting and prediction model.

[0044] 7) Determine the model and put it into use. Use the coercivity detection device to measure the surface coercivity of the metal plate to be tested, and calculate the thickness of the decarburized layer on the metal surface through the mathematical model to achieve a fast and efficient prediction of the decarburized layer thickness.

[0045] Take another piece of the metal material to make a sample, and select 5 test points on the sample. The sample to be tested with the test points marked is shown in Figure 4 ; Determine the surface hysteresis loop of the metal at 5 test points and calculate the coercive force; compare the coercive force with the mathematical model between the coercive force of the corresponding metal and the thickness of the decarburized layer to determine the thickness of the decarburized layer. The coercive force data and the calculation results of the decarburized layer thickness are listed in Table 1.

[0046] 8) Detection and verification of the decarburization layer thickness of the metal material.

[0047] At the same time, in order to verify the test results, the sample is cut in the test area, and the cut samples are Figure 5 , prepare the metallographic specimen, and after cold mounting, rough grinding, fine grinding, metallographic specimen polishing, chemical etching and other processes, use a metallographic microscope to calibrate the thickness of the decarburized layer at each test point in the test area. Each test point is measured 5 times and the average value is taken. The metallographic photos and measured values ​​of the 5 test points are shown in Figure 6-10, the calculation results are recorded in Table 1:

[0048] Table 1 Test results of two test methods for Q235B metal in this embodiment

[0049]

[0050] Note: The test error is calculated as (|ab|) / b.

[0051] It can be seen from Table 1 that the error between the thickness of the decarburized layer detected by the method of the present invention and the thickness of the decarburized layer detected by the metallographic method is kept within 4%.

Claims

1. A method for detecting the thickness of a decarburized layer based on coercivity detection, characterized in that: It includes the following: (1) Establishment of mathematical model between coercivity and decarburized layer thickness; (2) Determine the thickness of the decarburized layer by testing the coercivity; Among them, the method for establishing the mathematical model between coercivity and decarburized layer thickness is: Decarburized layers are prepared on different metal sheets, and different thicknesses of decarburized layers are produced on the surfaces of different samples by heating, heat preservation and cooling; the thickness of decarburized layers on the surfaces of different samples is observed by metallographic method; the hysteresis loop of the sample surface is measured, and the coercive force is calculated; the thickness of the decarburized layers on the surfaces of different samples is matched with the corresponding coercive force values, and the two are numerically fitted to establish a mathematical model between the coercive force and the thickness of the decarburized layers of the corresponding metal sheets; The method for determining the thickness of the decarburized layer by detecting the coercive force is: The hysteresis loop of the metal surface to be tested is measured and the coercive force is calculated; the coercive force is compared with the mathematical model between the coercive force of the corresponding metal and the thickness of the decarburized layer to determine the thickness of the decarburized layer.

2. A method for detecting the thickness of a decarburized layer based on coercivity detection as claimed in claim 1, characterized in that: The metal plate is heated, kept warm and cooled so that decarburized layers of different thicknesses are produced on the surfaces of different samples. The heating rate and temperature peak, the holding time and the cooling speed all need to be determined experimentally according to the type of plate material.

3. A method for detecting the thickness of a decarburized layer based on coercivity detection as claimed in claim 2, characterized in that: The coercive force measuring device composed of a power supply, a signal generating module, a detection probe, a signal acquisition module, a signal processing module and an oscilloscope is used to measure the hysteresis loop on the surface of the sample and calculate the coercive force; When measuring the coercive force, the signal excitation module of the coercive force measuring device excites a sinusoidal current signal, which is amplified and sent to the detection probe to excite a magnetic field on the surface of the tested sample, detect its hysteresis loop, and calculate its coercive force value.

4. A method for detecting the thickness of a decarburized layer based on coercivity detection as claimed in claim 3, characterized in that: The metallographic method is used to observe the thickness of the decarburized layer on the surface of different samples. It is necessary to determine a certain number of sample points on the surface of the metal plate on which the decarburized layer has been prepared, cut the sample at the sample point position, and observe the cut section through a metallographic microscope. The thickness of the decarburized layer at each sample point is calibrated at the obvious decarburization position, and the average value of the decarburized layer thickness at all sample points of the same plate sample is taken as the decarburized layer thickness on the surface of the plate.

5. A method for detecting the thickness of a decarburized layer based on coercivity detection as claimed in claim 4, characterized in that: The method of making a one-to-one correspondence between the decarburized layer thickness on the surface of different samples and the corresponding coercive force values, and performing numerical fitting on the two to establish a mathematical model between the coercive force and the decarburized layer thickness is to find the intrinsic mathematical relationship between the decarburized layer thickness and the coercive force size through linear regression numerical fitting after obtaining a certain amount of coercive force sample values ​​corresponding to decarburized layers of different thicknesses, and to construct a mathematical model between the coercive force and the decarburized layer thickness.

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