TWI614495B - Steel cleanliness evaluation method and cleanliness evaluation device - Google Patents

Abstract

The method for evaluating the cleanliness of steel according to the present invention is characterized by the following steps: an inclusion detection size setting step, which sets the inclusions in the steel to be detected by ultrasonic flaw detection, which are perpendicular to the rolling direction of the steel The minimum length d _{3 in the} width direction; the ultrasonic flaw detection step, which uses an ultrasonic detector 10 forming a spot-focused ultrasonic beam, transmits or receives ultrasonic beams to the steel by the water immersion flaw detection method, and scans the surface and rolls The two-dimensional scanning is carried out on the steel in a parallel direction; the one-dimensional processing step obtains the two-dimensional distribution of the ultrasonic reflection signal level corresponding to the two-dimensional scanning surface, and performs the one-dimensional processing on the two-dimensional distribution And the evaluation step, in the one-dimensional ultrasonic reflection signal level, the sum of the lengths in the rolling direction of the area equal to or greater than the signal level of length d _{3 is} obtained and evaluated.

Description

Steel cleanliness evaluation method and cleanliness evaluation device

The present invention relates to a cleanliness evaluation method and a cleanliness evaluation device for steel materials, and particularly to a cleanliness evaluation method and a cleanliness evaluation device for high-cleanliness steel materials requiring high fatigue characteristics.

It is known that in order to achieve high fatigue characteristics of products manufactured by processing steel materials, it is necessary to reduce the non-metallic inclusions (hereinafter referred to as inclusions) that cause damage, so it is known that the inclusions inside the steel material are investigated . For example, Patent Document 1 describes a method for evaluating the cleanliness of steel materials by evaluating the inclusions using the ultrasonic flaw detection method. Among them, in Patent Document 1, as an evaluation method of inclusions, the number of inclusions whose reflected wave intensity is more than a certain level is evaluated.

[Prior Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2006-64569

However, the evaluation method proposed in Patent Document 1 has a problem that the evaluation accuracy of fatigue characteristics may not be sufficiently obtained. That is, the evaluation method proposed in Patent Document 1 is based on the higher the probability of inclusions being detected, the higher the probability that the inclusions actually exist on or near the surface of the steel (hereinafter referred to as the surface of the steel). tired The view of lower labor characteristics. However, the number of such inclusions does not necessarily correspond to the probability that the inclusions actually exist on the surface of the steel, and the cleanliness of the steel cannot be correctly evaluated in this evaluation method.

The present invention has been completed in view of the above circumstances, and an object thereof is to provide a cleanliness evaluation method and cleanliness evaluation device for a steel material that can accurately evaluate the cleanliness of the steel material.

In order to solve the above problems and achieve the objective, the method for evaluating the cleanliness of the steel of the present invention is a method for evaluating the cleanliness of the steel formed by rolling a cast steel sheet (slab) by ultrasonic flaw detection, which is characterized by It includes the following steps: Inclusion detection size setting step, which sets the minimum length d _{3 in} the width direction perpendicular to the rolling direction of the steel material for inclusions in the steel material to be detected by the ultrasonic flaw detection; Sonic flaw detection step, which uses an ultrasonic detector that forms a point-focused ultrasonic beam to transmit or receive ultrasonic beams to the above-mentioned steel by the water immersion flaw detection method, and to carry out on the above-mentioned steel in such a way that the scanning surface is parallel to the rolling direction Two-dimensional scanning; one-dimensional processing step, which obtains a two-dimensional distribution of the ultrasonic reflection signal level corresponding to the two-dimensional scanning surface, and performs one-dimensional processing on the two-dimensional distribution; and an evaluation step, which is In the above-mentioned one-dimensional ultrasonic reflection signal level, the rolling direction of the region equal to or greater than the signal level of the length d ₃ is obtained The total length and evaluation.

In addition, the method for evaluating the cleanliness of the steel of the present invention is characterized in that, before the ultrasonic flaw detection step, it further includes an ultrasonic detector setting step, which is cut in a direction perpendicular to the rolling direction of the steel sheet. When the area is set to S ₀ and the cross-sectional area in the direction perpendicular to the rolling direction of the steel material is set to S ₁ , the following formula (2) is satisfied when the following formula (1) is satisfied, but not satisfied In the case where the following formula (1) is satisfied, the beam diameter d ₂ of the ultrasonic beam of the ultrasonic probe used in the ultrasonic flaw detection step is set in a manner that satisfies the following formula (3).

In addition, the cleanliness evaluation method of the steel material of the present invention is characterized in that the length d _{3 is} set to 20 μm or less in the step of setting the size of detection of inclusions.

In order to solve the above problems and achieve the objective, the cleanliness evaluation device of the steel of the present invention evaluates the cleanliness of the steel formed by rolling the cast steel sheet by ultrasonic flaw detection, and is characterized by using the forming point The ultrasonic detector focused on the ultrasonic beam transmits or receives the ultrasonic beam to the above-mentioned steel by the water immersion flaw detection method, and performs a two-dimensional scan on the above-mentioned steel in such a way that the scanning surface is parallel to the rolling direction The two-dimensional distribution of the ultrasonic reflection signal level corresponding to the one-dimensional scanning surface, and performing one-dimensional processing on the two-dimensional distribution, in the above one-dimensional ultrasonic reflection signal level, it is found to be equivalent to the length The sum of the lengths in the rolling direction of the area above the signal level of d ₃ is evaluated, and the length d ₃ is the rolling direction of the inclusions in the steel to be detected by the ultrasonic flaw detection and the steel The minimum length in the vertical width direction.

According to the present invention, the cleanliness is evaluated based on the sum of the lengths of the inclusions in the extending direction, so the correspondence with the probability that the inclusions actually exist on or near the surface of the steel becomes good, so that the steel can be evaluated Cleanliness.

1‧‧‧ Cleanliness evaluation device

10‧‧‧point-focus ultrasonic detector

20‧‧‧Control Department

A ₀ , A ₁ ‧‧‧ reflected signal

A _th ‧‧‧ detection threshold

C‧‧‧Supersonic speed of sound in water

D‧‧‧Oscillator diameter

d ₀ ‧‧‧Diameter

d ₁ ‧‧‧Length (short diameter)

d ₂ ‧‧‧beam diameter

d ₃ ‧‧‧Inclusion detection size

F‧‧‧ Focus distance in water

f‧‧‧Ultrasonic frequency law

I‧‧‧Length

L‧‧‧Length (long diameter)

L’ 1, L’ 2, L’ 3 length

p‧‧‧Flaw detection distance

S ₀ ~ S ₃ ‧‧‧Cross-sectional area

FIG. 1 is a schematic diagram showing the cast steel sheet rolled into a round steel bar (steel).

FIG. 2 is a schematic diagram showing a point-focused ultrasonic detector used in ultrasonic inspection using a water immersion inspection method.

FIG. 3A is a schematic diagram showing the relationship between the inclusions and the beam focusing portion, and is a diagram showing the case where the length of the inclusions exceeds the beam diameter.

FIG. 3B is a schematic diagram showing the relationship between the inclusions and the beam focusing portion, and showing the case where the length of the inclusions is less than the beam diameter.

FIG. 4A is a schematic diagram showing the presence of one inclusion in the steel.

FIG. 4B is a schematic diagram showing the presence of two inclusions in the steel.

FIG. 5 is a flowchart showing the content of the steel cleanliness evaluation method according to the embodiment of the present invention.

6 is a flowchart showing the contents of the evaluation condition setting procedure of the steel cleanliness evaluation method according to the embodiment of the present invention.

7 is a table showing the comparison results of the reflected signal (ultrasonic reflected signal) and the noise level for the four types of spot-focused ultrasonic detectors with different beam diameters under the same conditions as the actual ultrasonic flaw detection.

FIG. 8 is a view showing the method for evaluating the cleanliness of steel according to the embodiment of the present invention. Schematic diagram of injury spacing.

9 is a flowchart showing the content of the subject evaluation step in the cleanliness evaluation method of steel according to the embodiment of the present invention.

FIG. 10 is a diagram showing an example of a method for cutting a subject in the method for evaluating the cleanliness of steel according to the embodiment of the present invention.

FIG. 11 is a diagram showing an example of a flaw detection method showing a case where the flaw detection surface is a flat surface in the method for evaluating the cleanliness of steel according to the embodiment of the present invention.

FIG. 12 is a diagram showing an example of a flaw detection method showing a case where the flaw detection surface is a curved surface in the steel cleanliness evaluation method according to the embodiment of the present invention.

13 is a diagram showing an example of a two-dimensional image and one-dimensional data showing the level of ultrasonic reflection signals in the method for evaluating the cleanliness of steel according to an embodiment of the present invention.

FIG. 14 is a diagram showing an example of the evaluation of the total length of the detected inclusions in the cleanliness evaluation method of steel according to the embodiment of the present invention.

15A is a diagram showing the inclusions contained in the evaluation material 1 in an example of the method for evaluating the cleanliness of steel materials according to an embodiment of the present invention.

15B is a diagram showing the inclusions contained in the evaluation material 2 in an example of the method for evaluating the cleanliness of steel materials according to an embodiment of the present invention.

16A is a diagram showing a two-dimensional image and one-dimensional data of a two-dimensional scan of an evaluation material 1 in an embodiment of a method for evaluating the cleanliness of steel materials according to an embodiment of the present invention.

16B is a diagram showing a two-dimensional image of a two-dimensional scan of the evaluation material 2 and its one-dimensional data in an embodiment of a method for evaluating the cleanliness of steel materials according to an embodiment of the present invention.

17 is an example of the method for evaluating the cleanliness of steel according to the embodiment of the present invention In the table, the evaluation results of the method of the present invention and the evaluation results of the methods of the conventional technology are shown.

Hereinafter, the cleanliness evaluation method and cleanliness evaluation apparatus for steel materials according to the embodiments of the present invention will be described with reference to the drawings. Furthermore, the present invention is not limited to the following embodiments. In addition, the constituent elements of the following embodiments include a configuration that is simple and replaceable for those who know the art, or a configuration that is substantially the same.

The cleanliness evaluation method of the steel material of this embodiment is to evaluate the cleanliness of the steel material formed by rolling the cast steel sheet by ultrasonic flaw detection using the water immersion flaw detection method (hereinafter, referred to as the water immersion method) method. Here, in the following description, the background of the present invention will be described first, and then the specific content of the present invention will be described.

Generally, as a manufacturing process of steel materials, a steel sheet is cast first, and then the steel sheet is rolled by a rolling process to manufacture a steel material (for example, refer to Reference 1). Furthermore, sometimes the rolling step may be a plurality of steps, and sometimes heat treatment or surface treatment may be performed during this period.

Reference 1: Japanese Patent Laid-Open No. 2009-285698

In these manufacturing steps, it is estimated that inclusions are mixed into the steel sheet during casting. In addition, as shown in FIG. 1, at this time, the inclusions can be assumed to be substantially spherical. Then, as shown in the right figure of the figure, it is conceivable that the inclusions so mixed during casting will be extended after the rolling step. In this case, as shown in the figure, the cross-sectional area in the direction perpendicular to the rolling direction of the steel sheet during casting is set to S ₀ , and the cross-section in the direction perpendicular to the rolling direction of the round steel bar after rolling When the area is set to S ₁ and the diameter of the inclusions during casting is set to d ₀ , the shape of the inclusions of the rolled steel (such as a round steel bar), that is, the rolling direction (extending direction) of the inclusions The length (major diameter) L and the length (minor diameter) d _{1 in} the width direction perpendicular to the rolling direction are expressed by the following formula (4) and the following formula (5), respectively. Furthermore, the above-mentioned "rolling direction" refers to the direction in which the steel sheet is rolled and is parallel to the longitudinal direction of the steel material.

[Number 2] L = d ₀ . (S ₀ / S ₁ ) ... (4) d ₁ = d ₀ . (S ₁ / S ₀ ) ₁ / ² … (5)

In the above formula (4) and the above formula (5), it is assumed that the length L of the inclusion in the rolling direction (hereinafter referred to as the length L of the inclusion) is proportional to the rolling ratio S ₀ / S ₁ It is assumed that the length d ₁ (hereinafter, referred to as the width d _{1 of the} inclusion) in the width direction perpendicular to the rolling direction of the inclusions is circular in the cross section perpendicular to the width direction. In addition, in the above formula (4) and the above formula (5), it is assumed that the volume of the inclusions does not change before and after rolling.

In the case of ultrasonic flaw detection of steel products manufactured by rolling in this way, considering the detection ability and flaw detection efficiency of inclusions, it is usually carried out by using, for example, the water immersion method of the focus type detector described in Patent Document 1 described above. FIG. 2 shows a point-focused ultrasonic detector (hereinafter referred to as an ultrasonic detector) 10 used at this time, and a point-focused ultrasonic beam (hereinafter referred to as ultrasonic) formed by the ultrasonic detector 10 Beam).

It can be considered that the ultrasonic reflected signal from the inclusions (hereinafter referred to as the reflected signal) in the beam focusing section (focus, focusing area) shown in FIG. 2 is related to the beam cross-sectional area S _{2 of the} beam focusing section and the ultrasonic wave The ratio S ₃ / S ₂ of the cross-sectional area S ₃ of the inclusions contained in the beam is approximately proportional. 3A and 3B are cross-sectional views assuming the presence of inclusions in the beam focusing portion of the ultrasonic beam, and are shown parallel to the rolling direction to cut off the area of the steel where the inclusions are present, and viewed from above A schematic diagram of the condition of this cutting plane. Furthermore, here, it is assumed that the ultrasonic beam enters perpendicularly to the rolling direction of the steel material. If the ultrasonic beam is incident perpendicularly to the rolling direction in this way, the cross-sectional area of the inclusions in the beam focusing portion can be increased, so it is also advantageous for the detection of minute inclusions.

At this time, as shown in FIG. 3A, when the inclusion length L exceeds the beam diameter d ₂ (L> d ₂ ), no matter how the ultrasonic beam is aligned, a part of the inclusion will still exceed the ultrasonic beam. In this case, since the reflected signal from the steel can only cope with the cross-sectional area in the ultrasonic beam, there is a problem of not knowing the cross-sectional area of the entire inclusion. Furthermore, the above-mentioned "beam diameter" refers to the diameter of the beam focusing portion of the ultrasonic beam.

On the other hand, as shown in FIG. 3B, if an ultrasonic beam whose length L is equal to or smaller than the beam diameter d ₂ (L ≦ d ₂ ) is used, the entire inclusion can be enclosed in the ultrasonic beam. However, in this case, since the beam cross-sectional area S ₂ becomes larger, the ratio S ₃ / S _{2 of the} beam cross-sectional area S ₂ to the cross-sectional area S ₃ of the inclusions becomes smaller, and the intensity of the reflected signal from the steel material becomes weaker, so There is a problem that the detection ability of inclusions is reduced.

In addition, as shown in FIG. 3A, when the beam diameter d _{2 is} reduced, although the cross-sectional area of the entire inclusion is unknown, as long as the cross-sectional area of the inclusion in the ultrasonic beam is more than a certain area (the cross-section of the inclusion) (The area is more than a certain degree relative to the cross-sectional area of the beam focusing section), and the inclusion can be detected. However, in this case, the following problems still exist when evaluating inclusions.

For example, when a steel bar, which is a round steel bar, is cut and processed and used as a machine part, etc., if inclusions are present on or near the surface of the machine part, it may cause breakage and deteriorate the life characteristics. Here, when thinking about the case of cutting the round steel bar shown in FIGS. 4A and 4B, if the position of the cutting surface is randomly set, the probability that the inclusions exist on the surface (or the inclusions are exposed when cutting Probability) becomes "total length of inclusions contained in the round steel rod ÷ length of the round steel rod". Therefore, as shown in FIG. 4A, the probability that an inclusion exists on the surface of a round steel rod (length I) containing only one inclusion (length L' ₁ ) becomes "L' ₁ / I". In addition, as shown in FIG. 4B, the probability that the inclusions exist on the surface of the round steel rod (length I) containing a total of two inclusions (lengths L ' ₂ and L' ₃ ) becomes "(L ' ₂ + L' ₃ ) / I ". Therefore, irrespective of the number of inclusions, the longer the length of the inclusions (hereinafter referred to as the total length), the higher the probability.

On the other hand, the conventional evaluation method proposed in Patent Document 1 only evaluates the number of detected inclusions, and it is considered that the more the number of inclusions increases, the more likely the inclusions are on the steel surface. High, and the lower the fatigue characteristics. Therefore, the evaluation method of Patent Document 1 has a problem that it cannot appropriately cope with the evaluation results of the inclusions and the evaluation of the fatigue characteristics. Therefore, in order to solve such a problem, the inventors of the present invention have devised a method and a device for evaluating the cleanliness of steel materials that can more accurately evaluate the cleanliness of steel materials. The content of the present invention will be described below.

The basic configuration of the apparatus for performing the method for evaluating the cleanliness of steel according to the embodiment of the present invention is the same as that shown in FIG. 2 above. That is, the cleanliness evaluation device 1 includes the ultrasonic probe 10 and the control unit 20. In addition, FIG. 2 only shows the configuration related to the present invention, and the other configurations are omitted.

The ultrasonic detector 10 forms an ultrasonic beam and performs ultrasonic flaw detection by the water immersion method. In addition, the control unit 20 controls the ultrasonic probe 10 and processes the reflected signal obtained by the ultrasonic probe 10. Specifically, the control unit 20 controls the CPU, disk device, memory device, input device, input A general computer composed of a device, a communication device, etc. functions as a means for performing the steps described later in the method for evaluating the cleanliness of steel materials in this embodiment.

As shown in FIG. 5, the cleanliness evaluation method of the steel material of this embodiment performs an evaluation condition setting step (step S1) and a subject evaluation step (step S2). Among them, as shown in FIG. 6, in the evaluation condition setting step, an inclusion detection size setting step (step S11), an ultrasound detector setting step (step S12), and a detection threshold setting step (step S13) are sequentially performed , And the setting step of the detection interval (step S14).

First, in the step of setting the detection size of the inclusions, the detection size of the inclusions is set. Among them, the detection size of inclusions refers to the size of the smallest (lower limit) inclusions in the steel to be detected by ultrasonic flaw detection. More specifically, it refers to the smallest length in the width direction perpendicular to the rolling direction of the smallest inclusion to be detected. In the following, the detection size of this inclusion is shown in d ₃ .

In this step, as shown in FIG. 1, it is assumed that the inclusions in the steel material are extended by rolling, and the detection size d ₃ of the inclusions corresponding to the width d _{1 of} the extended inclusions is set. Then, in the ultrasonic flaw detection step (see FIG. 9) in the latter stage, inclusions with d ₁ > d _{3 are} detected. Furthermore, since the smaller the value of the inclusion detection size d ₃ , the smaller the inclusions can be detected, so it is preferable to set it as small as possible. For example, the following Reference 2 discloses that a defect of 20 μm or less becomes the starting point of fatigue cracks. Therefore, in this step, it is preferable to set the inclusion detection size d ₃ to 20 μm or less.

Reference 2: Fujimori Hiroshi, etc., "Operation of cracks from internal defects in rolling fatigue of high-carbon chromium bearing steel", Iron and Steel, Japan Iron and Steel Association, 2008, Vol.94, No.1 , P13-20

Next, in the setting procedure of the ultrasonic detector, the ultrasonic detector 10 used for ultrasonic flaw detection is set. In this step, after considering the extension caused by the rolling of the inclusions mixed during casting, the conditions for ultrasonic flaw detection of the ultrasonic beam are determined. In the following, the numerical formula for determining the conditions for ultrasonic flaw detection will be described.

First, when the oscillator diameter of the ultrasonic detector 10 is set to D, the focal distance in water is set to F, the ultrasonic speed of sound in water is set to C, and the ultrasonic frequency is set to f, the ultrasonic detector 10 has The beam diameter d ₂ can be expressed by the following formula (6).

Furthermore, the beam diameter d _{2 of} the ultrasonic beam is basically unchanged even when the ultrasonic beam is incident on the steel material. At this time, the beam cross-sectional area S _{2 of the} beam focusing section can be expressed by the following formula (7).

On the other hand, regarding the inclusions, the length L of the inclusions is based on the above formula (4) and the above formula (5), and using the width d _{1 of the} inclusions and the rolling ratio S ₀ / S ₁ , the following formula (8) shown.

[Numerical formula 5] L = d ₁ . (S ₀ / S ₁ ) ^3/2 … (8)

At this time, when the inclusions are located in the center of the ultrasonic beam cross section (refer to FIGS. 3A and 3B), the cross-sectional area S ₃ of the inclusions contained in the ultrasonic beam can be expressed as the following formula (9) and the following formula (10) As shown. Furthermore, in the following formula (9) and the following formula (10), when L> d ₂ , the cross section of the inclusions in the ultrasonic beam is approximately rectangular, and in the case of L ≦ d ₂ , The cross section of the inclusions in the ultrasonic beam is approximated as an ellipse.

[Equation 6] In the case of L> d ₂ , S ₃ = d ₁ . d ₂ … (9) When L ≦ d ₂ , S ₃ = (π / 4) d ₁ . L… (10)

Next, consider the case where the ratio S ₃ / S ₂ of the cross-sectional area S ₂ of the ultrasonic beam to the cross-sectional area S ₃ of the inclusions contained in the ultrasonic beam. If the reflection signal A _{0 in} the case where the ultrasound beam is reflected from the ultrasound beam as a whole is used, the reflection signal A ₁ that can be obtained from the inclusions during the ultrasound flaw detection can be expressed by the following formula (11). In addition, in the following formula (11), it is assumed that the reflectance of the reflected signals A ₁ and A ₀ per unit area is the same.

[Equation 7] A ₁ = A ₀ . (S ₃ / S ₂ )… (11)

Here, in ultrasonic flaw detection, it suffices to be able to detect the reflected signal A ₁ corresponding to the inclusion detection size d ₃ or more set in the inclusion detection size setting step described above. a quasi noise detection bit is set A _n, and the margin for the detection of the value (SN ratio) is set to α, as the reflected signal lines A ₁ represented by the following formula (12) of the values shown.

[Equation 8] A ₁ ≧ α. A _n … (12)

According to the above description, the conditions required for the beam diameter d ₂ of the ultrasonic beam in order to detect inclusions with d ₁ ≧ d ₃ are expressed as the following formula (8) to (12) 13) and the following formula (14).

Here, consider the case of the above-described formula (13), and in the above-described formula (14) A _n of the noise level. As shown in FIG. 7, the inventors conducted reflection signal A ₀ and noise level for the four types of ultrasonic detectors 10 with different beam diameters d _{2 of the} ultrasonic beam under the same conditions as the actual ultrasonic flaw detection comparison of A _n. It can be seen from the "reciprocal of SN ratio" shown in the same figure that it has nothing to do with the beam diameter d ₂ as long as An ≒ 0.01. A _{0 is} enough. Considering this factor, the above formula (13) and the above formula (14) become the following formula (15) and the following formula (16).

Also, the margin value α needs to be at least α ≧ 2. In this case, the above formula (15) and the above formula (16) become the following formula (17) and the following formula (18).

In addition, it is more preferable that the margin value α is set to α ≧ 5. In this case, the above formula (17) and the above formula (18) become the following formula (19) and the following formula (20).

According to the above description, in this step, the beam diameter d ₂ of the ultrasonic beam of the ultrasonic probe 10 used in the ultrasonic flaw detection step described later is set to satisfy the above formula (17) or the above formula (18) (compared Preferably, it is the above formula (19) or the above formula (20)). In other words, the beam diameter d _{2 is} set such that the following formula (2) is satisfied when the following formula (1) is satisfied, and the following formula (3) is satisfied when the following formula (1) is not satisfied. .

Next, in the detection threshold setting step, the detection threshold A _{th of the} inclusions is set. Here, as long as the reflected signal A ₁ of the inclusion equivalent to the inclusion detection size d ₃ can be detected, the detection threshold A _th is set to satisfy the above expression (8) to the expression (11) The following formula (21) and the following formula (22).

Next, in the step of setting the inspection pitch, the inspection pitch p is determined. The flaw detection interval p can be set based on the beam diameter d ₂ of the ultrasonic beam and set to be free of flaw detection. In consideration of, for example, the measurement points in FIG. 8, the flaw detection pitch p is set to satisfy the following equation (23).

The method for evaluating the cleanliness of the steel of this embodiment is to perform the evaluation condition setting step (step S1 in FIG. 5 and steps S11 to S14 in FIG. 6) as described above, and then perform the subject evaluation step (step S2 in FIG. 5). In the subject evaluation step, as shown in FIG. 9, the subject preparation step (step S21), the ultrasonic flaw detection step (step S22), the one-dimensional processing step (step S23), and the evaluation step are sequentially performed. Step S24).

First, in the preparation step of the subject, prepare the subject for ultrasonic flaw detection. In this step, specifically, for example, cutting of steel materials (round steel bars), surface smoothing processing, heat treatment for grain refinement, etc. are performed. In addition, in the cutting of steel materials, as shown in FIG. 10, the flaw detection surface is set parallel to the rolling direction. Also preferably, After the two-dimensional immersion flaw detection is carried out efficiently thereafter, as shown in the figure, the test object is cut in a fractional form where the flaw detection surface becomes a flat surface.

Secondly, in the ultrasonic flaw detection step, ultrasonic flaw detection is implemented. In this step, the water immersion method is used for flaw detection of the ultrasonic probe 10 with point focusing and efficiency. In addition, in this step, the ultrasonic detector 10 set in the ultrasonic detector setting step is used, and the ultrasonic beam is transmitted or received according to each detection interval p set in the detection interval setting step, and one by one according to each position The reflected signal A _{1 is} detected, thereby generating a two-dimensional image (two-dimensional distribution) of the reflected signal level (reflected signal intensity).

In addition, the above two-dimensional image is generated so that the rolling direction of the steel material can be understood. In addition, as a specific method of ultrasonic flaw detection in this step, in the case where the flaw detection surface is a flat surface, as shown in FIG. 11, for example, a C-scan flaw detection method described in Reference 3 is preferably used.

Reference 3: Japanese Patent Laid-Open No. 2008-261889

In addition, when the flaw detection surface is a curved surface, for example, as shown in FIG. 12, it is preferable to perform two-dimensional scanning by combining the rotation of the axial direction and the movement in the direction parallel to the axial direction.

In this way, in this step, the ultrasonic detector 10 forming the ultrasonic beam is used and the ultrasonic beam is transmitted or received to the object, that is, the steel by the water immersion method, and the scanning surface is parallel to the rolling direction on the steel Perform a two-dimensional scan.

Next, in the one-dimensional processing step, the one-dimensional processing of the rolling direction of the two-dimensional image of the reflected signal level is performed. In this step, a two-dimensional image of the reflected signal level corresponding to the two-dimensionally scanned surface in the above ultrasonic flaw detection step is obtained, and the two-dimensional image is subjected to one-dimensional processing. In this step, specifically as shown in Figure 13, in In the designated area on the two-dimensional image, the data is one-dimensionalized by collecting the maximum value in the direction perpendicular to the rolling direction of the object, that is, the steel.

Finally, in the evaluation step, the total length of the inclusions detected from the subject is evaluated. In this step, among the reflected signal levels that have been one-dimensionalized in the one-dimensional processing step described above, an area equal to or greater than the signal level (detection threshold A _th ) corresponding to the detection size d ₃ of the inclusions The total length of the rolling direction is evaluated. That is, in this step, specifically, as shown in FIG. 14, for the one-dimensional reflected signal level, the inclusions set in the detection threshold setting step are counted as the detection threshold A _th or more Quantity, and multiply the data spacing in the rolling direction to calculate the total length of the inclusions. Moreover, in this step, it is evaluated that the longer the total length of the inclusions, the higher the probability that the inclusions exist on the surface of the steel material.

Furthermore, in one of the dimensionalization processing steps and evaluation steps shown in FIG. 13 and FIG. 14 above, from the two-dimensional image of the reflection signal level obtained from the entire steel material, the reflection signal level is specified (acquired) to be large One-dimensional processing and evaluation of the overall length of the area. However, it is also possible not to perform such area designation, but to perform one-dimensional processing and total length evaluation at a time in a wide range. In the former case, it has the advantage of being able to obtain the total length value of each area, and in the latter case, it has the advantage of reducing the effort of the designated area.

According to the cleanliness evaluation method and cleanliness evaluation device 1 of the steel material of the present invention described above, since the cleanliness is evaluated on the basis of the total length of the extending direction of the inclusions, it is different from the probability that the inclusions actually exist on the surface of the steel Correspondence becomes good, so that the cleanliness of the steel can be correctly evaluated.

Examples

Hereinafter, the present invention will be described more specifically using examples as examples. In this example, the method of the present invention and the method of conventional technology (Patent Document 1) were used to evaluate the cleanliness of a round steel bar rolled at a rolling reduction ratio S ₀ / S ₁ = 10, and the results were compared .

In the method of the present invention, it is assumed that the inclusion detection size d ₃ = 10 μm, and the beam diameter d ₂ = 0.20 mm described in No. 2 of FIG. 7 is used as the ultrasonic detector. In addition, set the margin value α = 5, and set the detection interval p = 0.1mm. As a result, as shown below, the condition of the above equation (19) in the ultrasound probe setting step becomes satisfied.

In the ultrasonic flaw detection step of this embodiment, in the case of flaw detection in the area of 20 mm × 10 mm, as shown in FIG. 15A and FIG. 15B, numerical experiments on flaw detection of two types of evaluation materials with inclusions distributed were performed. In addition, here, it is assumed that the inclusions are formed into the shapes shown in the above formula (4) and the above formula (5) by rolling.

16A and 16B show the results of performing the ultrasonic flaw detection step as described above, and then going through the one-dimensional processing step and evaluation step to evaluate the total length of the inclusions. Here, in the two-dimensional image of the reflected signal level, the reflected signal becomes the level shown in the above equation (11). In addition, set the noise level A _n ≒ 0.01. A ₀ , and the detection threshold A _th are as follows.

[Formula 17]

Next, FIG. 17 shows the evaluation result of the method of the present invention and the evaluation result of the method of the conventional technology. In the evaluation material 1 and the evaluation material 2, as shown in FIGS. 4A and 4B described above, the probability of the inclusion of the evaluation material 1 being present on the surface of the steel material is relatively greater. In the method of the present invention, the probability that the inclusion is present on the surface of the evaluation material 1 whose total length is long is higher than that of the evaluation material 2, so the correspondence with the actual probability is good. On the other hand, according to the method of the prior art, the probability that the evaluation material 2 with a large number of inclusions is present on the surface of the evaluation material 2 is higher than that of the evaluation material 1, so it is a result that does not correspond to the actual exposure probability.

In the above, the cleanliness evaluation method and cleanliness evaluation device of the steel of the present invention have been described more specifically by the forms and examples for implementing the invention, but the substance of the present invention is not limited to these descriptions and must be based on the application The scope of the patent is explained extensively. In addition, various changes and changes based on these descriptions are naturally included in the substance of the present invention.

(Industry availability)

The present invention can accurately evaluate the cleanliness of steel materials, and therefore can be applied to a wide field centered on the manufacturing process of high-cleanliness steel materials.

Claims (4)

A method for evaluating the cleanliness of steel materials is a method for evaluating the cleanliness of steel products formed by rolling cast steel sheets by ultrasonic flaw detection, characterized in that it includes the following steps: Inclusion detection size setting step , Which sets the minimum length d _{3 in} the width direction perpendicular to the rolling direction of the steel material for the inclusions in the steel material to be detected by the ultrasonic flaw detection; the ultrasonic flaw detection step, which uses spot-forming focused ultrasonic beams Ultrasonic detector, which transmits or receives ultrasonic beams to the above-mentioned steel by means of water immersion flaw detection, and performs two-dimensional scanning on the above-mentioned steel in such a way that the scanning surface is parallel to the rolling direction; the one-dimensional processing step, which obtains The two-dimensional distribution of the ultrasonic reflection signal level corresponding to the two-dimensional scanning surface is subjected to one-dimensional processing on the two-dimensional distribution; and the evaluation step is included in the one-dimensional ultrasonic reflection signal level , The sum of the lengths in the rolling direction of the regions equal to or above the signal level of the length d ₃ is obtained and evaluated. The method for evaluating the cleanliness of steel according to claim 1, wherein, before the above-mentioned ultrasonic flaw detection step, it further includes an ultrasonic detector setting step, which sets the cross-sectional area in the direction perpendicular to the rolling direction of the above-mentioned steel sheet Is S ₀ , and when the cross-sectional area in the direction perpendicular to the rolling direction of the steel material is S ₁ , the following formula (2) is satisfied and the following is not satisfied when the following formula (1) is satisfied In the case of formula (1), the following formula (3) is satisfied, and the beam diameter d ₂ of the ultrasonic beam of the ultrasonic detector used in the ultrasonic detection step is set; [Formula 1]

The method for evaluating the cleanliness of steel materials according to claim 1 or 2, wherein the step of setting the size of the inclusion detection described above sets the length d ₃ to 20 μm or less. An apparatus for evaluating the cleanliness of steel products, which evaluates the cleanliness of steel products formed by rolling cast steel sheets by ultrasonic flaw detection, characterized by the use of ultrasonic detectors that form point-focused ultrasonic beams, Transmit or receive ultrasonic beams to the steel by water immersion flaw detection method, and perform a two-dimensional scan on the steel with the scanning surface parallel to the rolling direction to obtain the ultrasonic reflection signal corresponding to the two-dimensional scanned surface The two-dimensional distribution of the level, and the two-dimensional distribution is subjected to one-dimensional processing. In the above-mentioned one-dimensional ultrasonic reflected signal level, the area equal to or above the signal level of length d _{3 is} obtained The sum of the lengths in the rolling direction is evaluated, and the length d ₃ is the minimum length of the inclusions in the steel material to be detected by the ultrasonic flaw detection in the width direction perpendicular to the rolling direction of the steel material.