JP2006234387A - Evaluation method for flake defect of steel material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an evaluation method for a flake defect of a steel material allowing efficient and objective detection. <P>SOLUTION: A prescribed range within a test piece prepared from an evaluation objective steel is subjected to ultrasonic flaw detection while bringing flaw detection sensitivity of an ultrasonic flaw detection testing device into a prescribed sensitivity, a relation between a detected object area index by an ultrasonic flaw detection image and a reflected wave intensity is found in every of the detected objects detected by the ultrasonic flaw detection, a prescribed mixed area mixed with an inclusion and a flake defect, and a prescribed flake defect evaluation area that is an area on an orthogonal coordinate system excepting the mixed area are set on the orthogonal coordinate system using the first axis as the reflected wave intensity and using the second axis as the detected object area index, the relation between the detected object area index and the reflected wave intensity is exhibited as coordinates on the orthogonal coordinate system, in the every of the detected objects, and the evaluation objective steel is evaluated to be a normal material, based on the number of the detected objects existing in the flake defect evaluation area. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for evaluating white spot defects in steel materials, and more particularly, to a method for evaluating white spot defects in steel materials that enables precise evaluation by using a high-frequency ultrasonic flaw detection method.

  White spot defects are internal cracks induced by transformation stress generated in the cooling process of steel or internal strain accompanying hydrogen accumulation, and are likely to occur in alloy steel.

  In particular, in hypereutectoid steels such as SUJ2, white spot defects are predisposed to the occurrence of microcracks due to the pressure of supersaturated hydrogen in the net carbide precipitated by the Acm transformation. In addition, microcracks are generated in the carbide by various stresses.

  When there are white spot defects, the mechanical properties such as tensile and compressive properties of steel are often adversely affected. However, in consideration of productivity, it is not always possible to take a sufficient cooling process or soaking process, and it is practically difficult to completely prevent white spot defects.

  Therefore, from the viewpoint of quality assurance, it is necessary to inspect the steel slab in order to distinguish between a normal material without white spot defects and other steel materials.

  As a conventional method for evaluating white spot defects of steel materials, there is an evaluation by a macro structure test method defined in JIS standard (JIS G 0553) (for example, see Non-Patent Document 1).

The macro structure test method is a method for testing the macro structure of dendrites, ingot patterns, center segregation, porosity, pits, etc. by etching the steel cross section with hydrochloric acid, copper chloride ammonium, aqua regia, etc. . As a general rule, the roughness of the test surface is 30 to 3.5 μmRa as defined in JIS B0601. In the treatment method, the corrosive liquid is heated to about 70 to 80 ° C. and immersed for an appropriate time while observing the appearance of the tissue. When the tissue appears, the tissue is taken out from the solution, washed with water, neutralized, dried, and observed with the naked eye.
JIS standard (Standard number: JIS G 0553) Macro structure test method for steel

  However, the evaluation of white spot defects by the conventional macrostructure test method has the following problems.

  Since the inspection of the tissue by the conventional macro structure test method is a so-called sensitive inspection that is performed visually, the objectivity of the inspection result is poor. Moreover, since it will observe visually, it is also difficult to automate the inspection work.

  On the other hand, since the appearance (spreading) of white spot defects is often slight, it is considered that a precise white spot defect evaluation method using a high-frequency ultrasonic flaw detection method or the like is necessary.

  The present invention has been made in order to solve such a conventional problem, and it is possible to evaluate the white spot defect of a steel material with higher precision, and the white spot defect of the steel material that can be automated. It is intended to provide an evaluation method.

  In the invention described in claim 1 of the present application, the flaw detection sensitivity of the ultrasonic flaw detection test apparatus is set as a predetermined sensitivity, and ultrasonic flaw detection is performed for a predetermined evaluation range in a test piece made from the steel to be evaluated. For each detected object, the relationship between the detected object area index based on the ultrasonic flaw detection image and the reflected wave intensity is obtained, and the first axis is the reflected wave intensity, and the second axis is the detected object area index. A predetermined mixed area where inclusions and white spot defects are mixed on the coordinate system and a white spot defect evaluation area which is an area on the Cartesian coordinate system excluding the mixed area are set on the Cartesian coordinate system. In addition, the relationship between the reflected wave intensity for each detection object and the detection object area index is shown in coordinates, and the evaluation target steel is evaluated as a normal material based on the number of detection objects existing in the white spot defect evaluation region. This is a method for evaluating white spot defects of steel.

  Further, the invention according to claim 2 of the present application is a method for evaluating a white spot defect of a steel material, wherein the white spot defect evaluation region is a region on an orthogonal coordinate system excluding the evaluation exclusion region. is there.

  The invention according to claim 3 is a method for evaluating white spot defects in a steel material, in which the steel to be evaluated is steel rolled and / or forged at a forge ratio of 6 to 30. is there.

  Further, the invention according to claim 4 of the present application is the method for evaluating a white spot defect of a steel material, wherein the flaw detection frequency of the ultrasonic flaw detection test apparatus is 5 to 25 MHz.

  The invention described in claim 5 is a steel material characterized in that the ultrasonic flaw detection test apparatus uses a focus type probe having a beam diameter at the focal point in the test piece of φ0.5 to 1.5 mm. This is a method for evaluating white spot defects.

  Further, in the invention according to claim 6 of the present invention, the predetermined evaluation range in the test piece is a range in which the porosity existing range and the peripheral portion are excluded from the entire range of the inside of the test piece. This is a method for evaluating white spot defects.

  The invention according to claim 7 of the present application is a method for evaluating white spot defects of a steel material, characterized in that the steel to be evaluated is a high carbon chromium steel.

  The invention according to claim 8 of the present application is the method for evaluating a white spot defect of a steel material, wherein the test piece is subjected to an annealing treatment.

  According to the present invention, it becomes possible to detect white spot defects using an ultrasonic flaw detector, and therefore, it is possible to efficiently and objectively detect minor white spot defects. Thereby, it is possible to supply a high-quality steel material with few white spot defects.

  Hereinafter, a method for evaluating white spot defects of a steel material according to an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing an outer shape of a test piece used in a method for evaluating a white spot defect of a steel material according to an embodiment of the present invention.

  In producing the test piece 1 of the present embodiment, a billet of round bar steel is used, but is not limited to round bar steel. Moreover, the steel material used as the object of evaluation of the white spot defect of this embodiment is high carbon chromium steel. This is because high-carbon chromium steel is particularly susceptible to white spot defects. Of course, the steel material to be evaluated is not limited to this.

  Here, a billet prepared by forging Bloom is prepared. In general, in metal materials, countless microporosities (fine cavities) exist in the as-cast state, and inspection by ultrasonic flaw detection may be difficult. In contrast, specimens taken from billets that have been forged with bloom have reduced porosity and reduced adverse effects such as diffuse reflection of ultrasonic waves incident on the specimen, resulting in the best detection performance and more precision. Evaluation by simple ultrasonic flaw detection becomes possible.

  Here, the forging ratio is 6 or more and 30 or less, preferably 6 or more and 10 or less. When the forge ratio is 6 or more, the porosity is pressure-bonded and the porosity is reduced, and the detection accuracy of ultrasonic defects other than the porosity can be increased. Further, in a steel material in which the final forge ratio can be made larger than 30, white spot defects are also pressed and reduced, so that it is difficult to cause a problem.

  The test piece 1 cuts the billet in a plane perpendicular to the axial direction, and further forms two planes A and B that are substantially equidistant from the center axis of the billet and substantially parallel. And the test piece 1 is processed into the substantially rectangular parallelepiped shape which has an external dimension of WxDxH. Thereafter, heat treatment such as annealing is performed. Thereby, the structure | tissue in the test piece 1 is made into a fine and homogeneous structure | tissue, and a mechanical property is improved.

  Finally, planar polishing is performed on two substantially parallel planes A and B to make the surface of the test piece 1 smooth. The test piece 1 shown in FIG. 1 created in this way has little transmission loss on the surface A which is the ultrasonic incident surface, and enables accurate detection and evaluation of inclusions.

  Next, sensitivity calibration of the ultrasonic flaw detection test apparatus is performed. For example, in the calibration of sensitivity, the sensitivity at which the maximum reflected wave intensity obtained from a flat bottom hole of 11 mm in depth and φ1.5 mm in the STB-A22 specimen is 80% is used as the reference sensitivity, and the value obtained by increasing the reference sensitivity by 20 dB is used for flaw detection. Sensitivity is done.

  Next, ultrasonic testing is performed on the test piece prepared as described above by the water immersion ultrasonic testing method. A commercially available ultrasonic flaw detector is appropriately used as an apparatus for performing ultrasonic flaw detection in the white spot defect evaluation method for steel materials of the present embodiment. In the present embodiment, a focal probe is used as the ultrasonic flaw detector.

  FIG. 5 is a diagram illustrating an example of a UT image based on a difference in focal beam diameter. FIG. 5A shows a UT image with a beam diameter of about φ0.15 mm, and FIG. 5B shows a UT image with a beam diameter of about φ1.0 mm.

  Here, the beam diameter in the steel material of the focus type probe is preferably φ0.5 to 2.0 mm. This is because white spot defects tend to be dense, but when the beam diameter is smaller than 0.5 mm, they are detected separately, and as shown in FIG. 5A, individual defects are small. This is because there is a case where it is evaluated, so that it is not always properly distinguished from inclusion physical property defects. When the beam diameter is φ0.5 mm or more, whiteness defects are appropriately detected as shown in FIG. Further, when the beam diameter is larger than 2.0 mm, the detection ability is lowered, and it is difficult to evaluate a suitable white spot defect.

  The flaw detection frequency of the ultrasonic flaw detector is preferably 5 to 25 MHz. This is because when the flaw detection frequency is lower than 5 MHz, the detection ability of white spot defects is insufficient, and when the flaw detection frequency is higher than 25 MHz, the flaw detection volume is reduced.

  FIG. 2 is a schematic configuration diagram of an ultrasonic flaw detector provided with a focus-type probe. For ultrasonic flaw detection, an all-immersion water immersion ultrasonic flaw detector equipped with a focus type probe was used. The ultrasonic flaw detection apparatus includes a focus type probe 11, an ultrasonic flaw detection unit 12, a scanning unit 13, a personal computer (hereinafter referred to as “PC”) 14 having a microprocessor, and an imaging unit 15.

  For ultrasonic flaw detection, after setting the test piece 1 in the water tank, the test piece data, measurement sensitivity, focus position, gate position, and flaw detection pitch are input to the PC 14, and the focus type probe 11 is operated. To start.

  An ultrasonic wave is transmitted from the focus-type probe 11, hits an object such as a white spot defect, and the reflected wave is detected, and the reflected wave intensity and reflected waveform information (waveform output as graph, positive half wave) Desired information based on intensity, negative half-wave intensity, etc.). The scanning by the focus type probe 11 emits a plurality of ultrasonic waves and receives reflected waves at a predetermined interval of the test piece 1 (this interval is referred to as “flaw detection scanning pitch” or simply “scanning”). "Pitch").

  The reflected wave intensity is obtained by the relational expression shown as reflected wave intensity = MAX (positive half wave intensity, negative half wave intensity).

  FIG. 3 is a diagram showing a range inside a test piece to be ultrasonically detected.

  The range inside the test piece 1 to be evaluated for white spot defects is the gate portion 2, and the porosity existing range 3 near the center of the test piece 1 where porosity exists is excluded from the evaluation target after the end of scanning. Is done. The test piece 1 is subjected to a pressure forging treatment, and most of the porosity in the test piece 1 is pressure-bonded, and the range where the porosity exists is narrowed. However, the porosity in a predetermined range from the center of the test piece 1 still exists without being crimped. Therefore, by evaluating a white spot defect by excluding a predetermined range from the center of the test piece 1, the influence of large inclusions such as porosity during ultrasonic flaw detection can be reduced. It is possible to evaluate more appropriately.

  The porosity existing range 3 at the center of the test piece 1 varies depending on the concentration of carbon contained in the steel material. Therefore, the porosity existence range 3 at the center of the test piece 1 is clarified in advance, and a range to be excluded from the target range for evaluating inclusions is determined. The porosity existing range 3 is determined as follows.

  First, the position where the porosity exists is confirmed by grinding the test pieces having different carbon concentrations, and the result indicates the radial position where the vertical axis exists, that is, the distribution range [% D] ([ % D] is an abbreviation for DIAMETER, meaning diameter), and the horizontal axis is plotted in a graph representing C%, which is the carbon concentration of the test piece.

  And if the point which shows the porosity which exists in the position most distant from each center of each test piece from which each carbon concentration differs is connected, it will become a graph shown in FIG.

  From this graph, the porosity existence range 3, which is a range excluded from the inclusion evaluation target range, can be determined according to the carbon concentration of the test piece 1 as shown in the following (1) and (2).

Carbon concentration (C%) ≦ 0.4% Range excluded from center of test piece (%) = 30 (1)
Carbon concentration (C%)> 0.4% Range excluded from the center of the test piece (%) = 16.7 × (C%) + 23.3 (2)
That is, for each steel type, the range derived from the range of (1) or the formula of (2) is clarified as the porosity existing range 3 excluded from the inclusion evaluation target range, and in the range excluding the range where the porosity exists, The cleanliness of steel can be measured. The method according to the present embodiment is particularly effective if the material has a dangerous volume of about D / 4 from the surface, because the steel part used directly by the user as the material can be directly evaluated.

  Further, the peripheral portion of the test piece 1 including the dead zone 4, the outer peripheral portion 5, and the end portion 6 is excluded from the scanning range because it is difficult to accurately detect white spot defects due to a large amount of noise. . Note that the outer peripheral portion 5 of the test piece is, for example, approximately 90 to 100% D from the center of the test piece 1.

  FIG. 4 is a diagram illustrating an example of a measurement result of an ultrasonic flaw detector in the white spot defect evaluation method for steel according to the present embodiment. The vertical axis (y-axis) represents the square root (√AREA) (mm) of the detected object area based on the ultrasonic flaw detection (UT) image, and the horizontal axis (x-axis) represents the ultrasonic reflected wave intensity (%).

  In the present embodiment, the square root (√AREA) of the detected object area is used as an index representing the detected object area (detected object area index), but the index representing the detected object area is not limited to this. For example, the circle-converted diameter of the detected object can be used as an index representing the area of the detected object.

  Here, a region to be evaluated for white spot defects in the ultrasonic flaw detection region is a region C sandwiched between lines A and B in the figure.

  The reason why the area larger than the line A (evaluation exclusion area) in the figure is not subject to evaluation is that there is a certain degree of correlation between the intensity and the size of the white spot defect. It is because it is not considered much. The reason why the area smaller than the line B (mixed area) in the figure is not subject to the evaluation is that the area below this includes inclusions (black squares in the figure) and white defects (white circles in the figure). This is because they are mixed and difficult to identify. That is, it can be said that the region C (white spot defect evaluation region) in the figure is a region in which only white spot defects are detected.

  The method for obtaining the line A and the line B is as follows. First, an ultrasonic flaw detection apparatus used for white spot defect evaluation is used, and the flaw detection sensitivity of the ultrasonic flaw detection test apparatus is set to a predetermined sensitivity. An ultrasonic flaw detection test is performed on the point defect, and a relationship diagram between the reflected wave intensity and √AREA by the UT image is created.

  For white spot defects, for example, with a number of samples of about n = 10 to 20, the relationship between reflected wave intensity and √AREA based on the UT image is plotted, an area including them is created, and line A is determined.

  Similarly, for each (large) inclusion that provides a reflected wave intensity comparable to that of a white spot defect, the relationship between the reflected wave intensity and √AREA based on the UT image is obtained with, for example, n = about 10 to 20 samples. Plot to create an area that encompasses them and create line B.

  A larger number of n in the plot of white spot defects and (large) inclusions is preferable from the viewpoint of reliability. Further, the y-intercept, inclination, etc. of the lines A and B can be determined as appropriate depending on the accuracy of detection of white spot defects.

  In the present embodiment, the line A and the line B are straight lines. However, the present invention is not limited to this, and a form such as a curve can be appropriately taken depending on the evaluation method.

  Then, the results of the ultrasonic flaw detection test are plotted in the figure, and when the number of white spot defects in the region C is 0, the steel material is regarded as a normal material. In the present embodiment, a steel material having zero white spot defects is used as a normal material. However, the present invention is not limited to this, and a normal material may be used when the number of white spot defects is a predetermined number or less.

  As described above, according to the evaluation method for white spot defects of steel materials according to the embodiment of the present invention, in order to detect white spot defects by the ultrasonic flaw detection method regardless of the sensitive inspection, Objective evaluation of defects is possible.

  Next, a method for evaluating a white spot defect of a steel material according to an embodiment of the present invention will be described in more detail with reference to examples. However, the evaluation method of the white spot defect of the steel material of the present invention is not limited to the following examples.

  First, a small piece of 80mm length is taken from the end of each SUJ2φ167mm billet, shaped into W167mm x D80mm x H47mm by roughing, and subjected to 800 ° C spheroidizing annealing, and multiple ultrasonic flaw detection tests Finish in a piece (see Figure 1). The outer shape of the test piece is finished into a substantially rectangular parallelepiped of W167 mm × D80 mm × H45 mm.

Then, a water immersion ultrasonic flaw detection test with a flaw detection frequency of 15 MHz is performed. For the flaw detection conditions, a probe having an ultrasonic beam diameter of about 1 mm at the focal point in the steel was used. The sensitivity at which the maximum reflected wave intensity obtained is 80% is defined as the reference sensitivity, and the value obtained by sensitizing the reference sensitivity by 20 dB is defined as the flaw detection sensitivity. Further, the flaw detection pitch is a plane scan of 0.2 mm × 0.2 mm, and the focal point in the steel is 22.5 mm deep (the center depth of the evaluation range). An evaluation range is a range of W _T (= W _T1 + W _T2 ) 83.5 mm × D _T 70 mm × H _T 25 mm, which is located in the center of the ultrasonic flaw detection specimen and excludes the dead zone and the end. (See FIG. 3). The total weight of the evaluation range in this case is about 1.15 kg in the case of SUJ2.

In the case of S53C, W _T (= W _T1 + W _T2 ) 96.5 mm × D _T 70 mm × H _T, which is located in the center of the ultrasonic flaw detection test piece and excludes the dead zone and the end. A range of 25 mm is set as the evaluation range. In this case, the total weight of the evaluation range is about 1.33 kg.

Further, in the case of SUS440C, W _T (= W _T1 + W _T2 ) 83.5 mm × D _T 70 mm, which is located in the center of the ultrasonic flaw detection test piece and excludes the dead zone and the end, as in SUJ2. The range of xH _T 25 mm is set as the evaluation range. In this case, the total weight of the evaluation range is about 1.15 kg.

  Next, the relationship between the reflected wave intensity for each detected object and √AREA is obtained from the result of the ultrasonic flaw detection test, and the relationship diagram shown in FIG. 4 is created. In the present embodiment, √AREA is a value obtained by a threshold method using a reflected wave intensity of 30% as a threshold. Also, the line A in FIG. 4 is y = 0.074x−1.102, and the line B is y = 0.031x−0.151. Then, an area between 0.074x−1.102 ≦ y ≦ 0.031x−0.151, which is an area sandwiched between the line A and the line B, is defined as an area C.

  The region C is a region where only white spot defects are detected. Within the evaluation range of the test piece 1, a steel material in which the number of 15 MHz UT defects in the region C range of FIG.

  The following two cases were set as test conditions. The first is a plurality of continuously manufactured billets, starting with a billet that has been in an unsteady operation, withdrawing eight billets, cutting the end positions of the eight billets as test pieces, and ultrasonic flaw detection A test was conducted (case 1 in FIG. 7). The second is a plurality of billets manufactured continuously, with 4 billets by steady operation and 3 after unsteady operation, cutting the end positions of 7 billets as a test piece. Then, an ultrasonic flaw detection test was conducted (case 2 in FIG. 7).

  And the detection result of the white spot defect of a present Example was compared with the evaluation result by a macro test. FIG. 8 is a diagram illustrating a result of comparing the detection result of the white spot defect of this example with the evaluation result by the macro test.

  In the macro test evaluation, A, B, and C are indices related to the number of white spot defects by φ167 billet T plane observation, A is less than 50, B is 50 to 200, and C is 200 or more. (The average score of rank B is 100). In addition, in the evaluation of the white spot defect of this example, ○ and × indicate that ○ indicates a normal material and × indicates the other.

  As shown in the figure, the evaluation of the white spot defect in this example is consistent with the evaluation A of the number of white spot defects by the macro test. Sex was proved.

It is a figure which shows the external shape of the test piece used for the evaluation method of the white spot defect of the steel materials of this embodiment. It is a schematic block diagram of the ultrasonic flaw detector provided with the focus type probe of this embodiment. It is a figure which shows the range inside the test piece by which ultrasonic testing of this embodiment is carried out. It is a figure which shows an example of the measurement result of the ultrasonic flaw detector in the white spot defect evaluation method of the steel material of this embodiment. It is a figure which shows the example of the UT image | video by the difference in the beam diameter of a focus. It is a graph showing the relationship between the maximum distribution range of porosity and carbon concentration. It is a figure which shows the sampling method of the test piece used for a present Example. It is a figure which shows the result of having compared the detection result of the white spot defect of a present Example, and the evaluation result by a macro test.

Explanation of symbols

1. Test piece 2. Gate part 3. Porosity presence range 4. Dead band 5. Outer part 6. End part 11. Focal probe 12. Ultrasonic flaw detection unit 13. Scanning unit 14. PC
15. Visualization unit

Claims (8)

With the flaw detection sensitivity of the ultrasonic flaw detection test apparatus as a predetermined sensitivity, ultrasonic flaw detection is performed for a predetermined evaluation range in a test piece made from steel to be evaluated,
For each detected object detected by the ultrasonic flaw detection,
Detected object area index by ultrasonic flaw detection image,
Find the relationship with reflected wave intensity,
On an orthogonal coordinate system with the first axis as the reflected wave intensity and the second axis as the detected object area index,
A predetermined mixed area in which inclusions and white spot defects are mixed;
A white spot defect evaluation area that is an area on the orthogonal coordinate system excluding the mixed area, and
On the orthogonal coordinate system, the relationship between the reflected wave intensity for each detection object and the detection object area index is shown in coordinates,
A method for evaluating a white spot defect in a steel material, wherein the steel to be evaluated is evaluated as a normal material based on the number of detected objects existing in the white spot defect evaluation region.   The method for evaluating a white spot defect of a steel material according to claim 1, wherein the white spot defect evaluation area is an area on the orthogonal coordinate system excluding an evaluation exclusion area.   3. The method for evaluating white spot defects of a steel material according to claim 1, wherein the steel to be evaluated is steel rolled and / or forged at a forging ratio of 6 to 30. 4.   The method for evaluating a white spot defect in a steel material according to any one of claims 1 to 3, wherein a flaw detection frequency of the ultrasonic flaw detection test apparatus is 5 to 25 MHz.   5. The steel material according to claim 1, wherein the ultrasonic flaw detection apparatus uses a focus type probe having a beam diameter at a focal point in a test piece of φ0.5 to 2.0 mm. Evaluation method for white spot defects.   The steel material according to any one of claims 1 to 5, wherein the predetermined evaluation range in the test piece is a range in which the porosity existing range and the peripheral portion are excluded from the entire internal range of the test piece. Evaluation method for white spot defects.   The method for evaluating white spot defects of a steel material according to any one of claims 1 to 6, wherein the steel to be evaluated is high-carbon chromium steel. The method for evaluating a white spot defect in a steel material according to any one of claims 1 to 7, wherein the test piece is subjected to an annealing treatment.