JP6260582B2 - Steel material cleanliness evaluation method and cleanliness evaluation apparatus - Google Patents

Description

  The present invention relates to a steel material cleanliness evaluation method and a cleanness evaluation device, and more particularly to a cleanliness evaluation method and a cleanness evaluation device for high cleanliness steel materials that require high fatigue characteristics.

  It is known that non-metallic inclusions that cause damage (hereinafter referred to as inclusions) must be reduced in order to achieve high fatigue characteristics of products manufactured by processing steel materials. In the past, investigations have been made on the inclusions inside the steel material. For example, Patent Document 1 describes a method for evaluating the cleanliness of a steel material by evaluating inclusions using an ultrasonic flaw detection method. Here, in Patent Document 1, as an evaluation method for inclusions, the number of inclusions whose reflected wave intensity is a certain level or more is evaluated.

JP 2006-64569 A

However, the proposed evaluation method in Patent Document 1, because a flaw detection using a point focused ultrasound beam, as shown in FIG. 15, when the beam diameter of the point focused ultrasound beams was d _2, It is necessary to set the flaw detection pitch p so as to satisfy the following formula (5).

Therefore, the proposed evaluation method in Patent Document 1, in order to reduce the beam diameter d ₂ of the point focused ultrasound beam in order to increase the detectability of inclusions, must be smaller flaw detection pitch p, as a result There is a problem that the number of measurement points per flaw detection area increases and the evaluation efficiency decreases.

  The present invention has been made in view of the above, and provides a steel material cleanliness evaluation method and a cleanliness evaluation device that can sufficiently obtain the detection ability of inclusions and are excellent in evaluation efficiency. Objective.

In order to solve the above-described problems and achieve the object, the steel material cleanliness evaluation method according to the present invention evaluates the cleanliness of a steel material formed by rolling a cast steel slab by ultrasonic flaw detection. a cleanliness evaluation method, the inclusions in the in the steel material to be detected by ultrasonic flaw detection, inclusions detection size for setting the minimum length d ₃ in the rolling direction perpendicular to the width direction of the steel Setting step, underwater focal length F, ultrasonic frequency f, underwater ultrasonic sound velocity C, transducer width in the focusing direction of the ultrasonic probe D _F , vibration in the non-focusing direction of the ultrasonic probe Lines used in the ultrasonic flaw detection, where the core width is D _NF , the cross-sectional area in the direction perpendicular to the rolling direction of the steel slab is S ₀ , and the cross-sectional area in the direction perpendicular to the rolling direction of the steel material is S ₁ For forming focused ultrasound beam The acoustic probe satisfies the following formula (1) and satisfies the following formula (3) when the following formula (2) is satisfied. When the following formula (2) is not satisfied, the following formula (4) is satisfied. An ultrasonic probe setting step for setting so as to satisfy the condition, and using the set ultrasonic probe so that the unfocused direction of the ultrasonic probe is parallel to the rolling direction of the steel material Corresponding to an ultrasonic flaw detection step in which an ultrasonic beam is transmitted / received to / from the steel material by a water immersion flaw detection method, and the steel material is two-dimensionally scanned so that the scanning surface is parallel to the rolling direction An evaluation step of acquiring a two-dimensional distribution of ultrasonic reflected signal levels to be evaluated and evaluating a portion having a signal level equal to or higher than the length d ₃ from the two-dimensional distribution.

Further, cleanliness evaluation method of the steel according to the present invention, in the inclusions detection size setting step, and setting the length d ₃ in 20μm or less.

In order to solve the problems described above and achieve the object, the steel material cleanliness evaluation apparatus according to the present invention evaluates the cleanliness of a steel material formed by rolling cast steel pieces by ultrasonic flaw detection. The underwater focal length is F, the ultrasonic frequency is f, the underwater ultrasonic velocity is C, the transducer width in the focusing direction of the ultrasonic probe is D _F , and the ultrasonic probe is When the width of the vibrator in the non-focusing direction is D _NF , the cross-sectional area in the direction perpendicular to the rolling direction of the steel slab is S ₀ , and the cross-sectional area in the direction perpendicular to the rolling direction of the steel material is S ₁ , An ultrasonic probe for forming a line-focused ultrasonic beam used for acoustic flaw detection satisfies the following formula (1), and satisfies the following formula (3) when the following formula (2) is satisfied: If the following formula (2) is not satisfied, the following formula (4) is set. Using the set ultrasonic probe, an ultrasonic beam is transmitted / received to / from the steel material by the water immersion flaw detection method so that the unfocused direction of the ultrasonic probe is parallel to the rolling direction of the steel material. However, the steel material is two-dimensionally scanned so that the scanning surface is parallel to the rolling direction, and a two-dimensional distribution of ultrasonic reflection signal levels corresponding to the two-dimensionally scanned surface is obtained. , characterized by evaluating the inclusions in the steel, the smallest portion of the signal level above which corresponds to the length d ₃ in the rolling direction perpendicular to the width direction of the steel the to be detected by the ultrasonic flaw And

  According to the present invention, in consideration of stretching due to rolling of inclusions mixed during casting, the condition of ultrasonic flaw detection by line-focusing ultrasonic waves is determined so as to be suitable for this, so that sufficient detection ability of inclusions is obtained. be able to. Also, by transmitting and receiving the ultrasonic beam so that the unfocused direction of the line-focused ultrasonic beam and the extension direction of the inclusions are parallel, the flaw detection pitch can be made coarse in the non-focused direction of the line-focused ultrasonic beam. Therefore, the number of measurement points per flaw detection area can be reduced and the evaluation efficiency can be improved.

Drawing 1 is a figure showing typically signs that a forged steel slab is rolled into round bar steel (steel material). FIG. 2 is a diagram schematically showing a point-focusing ultrasonic probe used for ultrasonic flaw detection by a water immersion flaw detection method. FIG. 3A is a diagram schematically illustrating the relationship between the inclusions and the beam converging unit, and is a diagram illustrating a case where the length of the inclusion exceeds the beam diameter. FIG. 3B is a diagram schematically illustrating the relationship between the inclusions and the beam converging unit, and is a diagram illustrating a case where the length of the inclusions is equal to or less than the beam diameter. FIG. 4 is a diagram showing a schematic configuration of a steel material cleanliness evaluation apparatus according to an embodiment of the present invention. FIG. 5 is a flowchart showing the contents of the steel material cleanliness evaluation method according to the embodiment of the present invention. FIG. 6 is a flowchart showing the contents of the evaluation condition setting step in the steel material cleanliness evaluation method according to the embodiment of the present invention. FIG. 7A is a diagram schematically illustrating the relationship between the inclusions and the beam converging unit, and is a diagram illustrating a case where the length of the inclusions is equal to or less than the beam width in the non-focusing direction. FIG. 7B is a diagram schematically illustrating the relationship between the inclusions and the beam converging unit, and is a diagram illustrating a case where the length of the inclusion exceeds the beam width in the non-focusing direction. FIG. 8 is a table showing a result of comparison of reflection signals (ultrasonic reflection signals) under conditions equivalent to those of an actual ultrasonic flaw detection for six types of ultrasonic probes having different specifications. FIG. 9 is a diagram schematically showing a flaw detection pitch in the steel material cleanliness evaluation method according to the embodiment of the present invention. FIG. 10 is a flowchart showing the contents of the object evaluation step in the steel material cleanliness evaluation method according to the embodiment of the present invention. FIG. 11 is a diagram illustrating an example of a specimen cutting method in the steel material cleanliness evaluation method according to the embodiment of the present invention. FIG. 12 is a diagram illustrating an example of a flaw detection method when the flaw detection surface is a flat surface in the steel material cleanliness evaluation method according to the embodiment of the present invention. FIG. 13 is a diagram showing an example of a flaw detection method when the flaw detection surface is a curved surface in the steel material cleanliness evaluation method according to the embodiment of the present invention. FIG. 14 shows the results of flaw detection using a conventional point-focused ultrasonic beam and a line-focused ultrasonic beam according to the present invention in an example of a steel material cleanliness evaluation method according to an embodiment of the present invention. It is a table. FIG. 15 is a diagram schematically showing a flaw detection pitch in a steel material cleanliness evaluation method using a conventional point-focused ultrasonic beam.

  Hereinafter, a steel material cleanliness evaluation method and a cleanliness evaluation apparatus according to embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. In addition, constituent elements in the following embodiments include those that can be easily replaced by those skilled in the art or those that are substantially the same.

  In the method for evaluating the cleanliness of steel materials according to the present embodiment, the cleanliness of steel materials formed by rolling cast steel slabs is determined by ultrasonic flaw detection using a water immersion flaw detection method (hereinafter referred to as a water immersion method). It is a method to evaluate. Here, first, the technology as the background of the present invention will be described first, and then the specific contents of the present invention will be described.

  Generally, as a manufacturing process of a steel material, a steel slab is first cast, and then the steel slab is extended by a rolling process to manufacture a steel material (for example, see Reference 1). In addition, there may be a plurality of rolling processes, and heat treatment or surface treatment may be performed during that time.

Reference 1: Japanese Unexamined Patent Application Publication No. 2009-285698

In these manufacturing processes, inclusions are presumed to be mixed into the steel slab at the time of casting. At that time, the inclusion is assumed to be approximately spherical as shown in the left figure of FIG. And it is assumed that the inclusion mixed at the time of casting is extended through a rolling process as shown in the right figure of the figure. In this case, the shape of inclusions in the rolled steel material (for example, round bar steel), that is, the length (major axis) L in the rolling direction (stretching direction) of the inclusions, and the length (short) in the width direction perpendicular to the rolling direction. As shown in the figure, the diameter d ₁ is the cross-sectional area in the direction perpendicular to the rolling direction of the steel slab during casting S ₀ , and the cross-sectional area in the direction perpendicular to the rolling direction of the round bar steel after rolling is S _1. When the diameter of inclusions at the time of casting is d ₀ , the following expressions (6) and (7) are obtained, respectively. The above-mentioned “rolling direction” is a direction in which a steel slab is rolled, and means a direction parallel to the length direction of the steel material.

Here, in the above formulas (6) and (7), the length L in the rolling direction of the inclusions (hereinafter referred to as the inclusion length L) is stretched in proportion to the rolling ratio S ₀ / S _1. It is assumed that the length d ₁ in the width direction perpendicular to the rolling direction of the inclusions (hereinafter referred to as inclusion width d ₁ ) is circular in the cross section perpendicular to the width direction. Moreover, in the said Formula (6) and said Formula (7), it is assumed that the volume of the inclusion is unchanged before and after rolling.

  In the case where ultrasonic flaw detection is performed on the steel material manufactured by rolling in this way, a focus type probe as described in Patent Document 1 described above, for example, in consideration of the detection ability and flaw detection efficiency of inclusions It is common to use the water infiltration method using FIG. 2 shows a point-focusing ultrasonic probe 110 used at that time and a point-focusing ultrasonic beam formed by the point-focusing ultrasonic probe 110.

Beam focusing unit shown in FIG. 2 (focal, focused area) reflected ultrasonic signals from the inclusions in (hereinafter, referred to as a reflected signal) and the beam cross-sectional area S ₂ in the beam focusing unit, a point focused in the ultrasonic beam It is considered to be approximately proportional to the ratio S ₃ / S ₂ to the cross-sectional area S ₃ of the inclusions included. 3A and 3B are cross-sectional views assuming that inclusions are present in the beam focusing portion of the point-focusing ultrasonic beam. A region where inclusions are present in a steel material is cut in parallel to the rolling direction, and the cutting is performed. It is a figure which shows typically a mode that the surface was observed from the top. Here, it is assumed that a point-focusing ultrasonic beam is incident perpendicular to the rolling direction of the steel material. When the point-focusing ultrasonic beam is incident perpendicularly to the rolling direction in this way, the cross-sectional area of the inclusions in the beam focusing portion increases, which is advantageous for detecting minute inclusions.

At this time, as shown in FIG. 3A, when the length L of the inclusion exceeds the beam diameter d ₂ (L> d ₂ ), a part of the inclusion is pointed no matter how the point focused ultrasonic beam is applied. It protrudes from the focused ultrasound beam. In this case, since the reflected signal from the steel material corresponds only to the cross-sectional area in the point-focused ultrasonic beam, there is a problem that the cross-sectional area of the entire inclusion is not known. The above-mentioned “beam diameter” means the diameter of the point focusing ultrasonic beam at the beam focusing portion.

On the other hand, as shown in FIG. 3B, if a point-focused ultrasonic beam is used in which the length L of the inclusion is equal to or less than the beam diameter d ₂ (L ≦ d ₂ ), the entire inclusion is contained within the point-focused ultrasonic beam. Can fit in. However, in this case, since the beam cross-sectional area S ₂ becomes large, the ratio S ₃ / S ₂ between the beam cross-sectional area S ₂ and the cross-sectional area S ₃ of the inclusion becomes small, and the intensity of the reflected signal from the steel material becomes small. Since it becomes weak, there exists a problem that the detection ability of inclusions will fall.

Incidentally, as shown in FIG. 3A, when reducing the beam diameter d _2, but do not know the cross-sectional area of the entire inclusions, point focused cross-sectional area of the inclusions in the ultrasound beam over a certain (beam focusing portion of the cross-sectional It is possible to detect inclusions if the cross-sectional area of the inclusions with respect to the area is greater than or equal to a certain value).

  However, in this case as well, as described above, it is necessary to reduce the flaw detection pitch p in order to improve the detection ability of inclusions (see FIG. 15 and the above equation (5)). As a result, the number of measurement points per flaw detection area is reduced. There was a problem that the evaluation efficiency decreased as the number increased. In order to solve such problems, the present inventors have devised a steel material cleanliness evaluation method and a cleanliness evaluation device excellent in evaluation efficiency. The contents of the present invention will be described below.

  FIG. 4 shows a basic configuration of an apparatus for performing a steel material cleanliness evaluation method according to an embodiment of the present invention. The cleanliness evaluation apparatus 1 includes a line-focusing ultrasonic probe (hereinafter referred to as an ultrasonic probe) 10 and a control unit 20. In FIG. 4, only the configuration related to the present invention is shown, and the other configurations are not shown.

  The ultrasonic probe 10 forms a line-focused ultrasonic beam (hereinafter referred to as an ultrasonic beam) and performs ultrasonic flaw detection by a water immersion method. Further, the control unit 20 controls the ultrasonic probe 10 and processes a reflection signal acquired by the ultrasonic probe 10. Specifically, the control unit 20 includes a general computer including a CPU, a disk device, a memory device, an input device, an output device, a communication device, and the like. It functions as a means for performing each step.

  As shown in FIG. 5, the steel material cleanliness evaluation method according to the present embodiment performs an evaluation condition setting step (step S1) and an object evaluation step (step S2). Among them, in the evaluation condition setting step, as shown in FIG. 6, the inclusion detection size setting step (step S11), the ultrasonic probe setting step (step S12), the detection threshold setting step (step S13), The flaw detection pitch setting step (step S14) is performed in this order.

First, in the inclusion detection size setting step, the inclusion detection size is set. Here, the inclusion detection size is the minimum (lower limit) size of inclusions in the steel material to be detected by ultrasonic flaw detection. More specifically, it means the length in the width direction perpendicular to the rolling direction of the minimum inclusion to be detected. The following shows that the inclusions detected size d _3.

In this step, as shown in FIG. 1, it is assumed that the inclusions in the steel material have been stretched by rolling, and the inclusion detection size d ₃ corresponding to the width d ₁ of the inclusion after the stretching is set. . Then, in the subsequent ultrasonic flaw detection step (see FIG. 10), inclusions that satisfy d ₁ > d ₃ are detected. Note that inclusions detected size d ₃ is, it is possible to detect a smaller value small inclusions, it is preferable that the possible small value. For example, in Reference Document 2 below, since it is shown that a defect of 20 μm or less becomes the starting point of a fatigue crack, in this step, the inclusion detection size d ₃ is preferably set to 20 μm or less.

Reference 2: Takefumi Fujimatsu et al., “Crack initiation behavior from internal defects in rolling fatigue of high-carbon chromium bearing steel”, Iron and Steel, Japan Iron and Steel Institute, 2008, Vol.94, No.1 , P13-20

  Next, in the ultrasonic probe setting step, the ultrasonic probe 10 used for ultrasonic flaw detection is set. In this step, conditions for ultrasonic flaw detection using an ultrasonic beam are determined in consideration of the beam width and transducer width in the focusing direction and the non-focusing direction. Hereinafter, equations for determining the ultrasonic flaw detection conditions will be described.

First, the beam width d _2F in the focusing direction at the beam focusing part of the ultrasonic beam is such that the transducer width in the focusing direction of the ultrasonic probe 10 is D _F , the underwater focal length is F, the underwater ultrasonic sound velocity is C, When the sound wave frequency is f, it can be expressed by the following equation (8).

Incidentally, focusing direction of the beam width d _2F of the ultrasonic beam, even if the ultrasonic beam is incident in the steel material, does not change basically.

Further, the beam width d _2NF in the non-focusing direction in the beam converging portion of the ultrasonic beam is such that the transducer width in the non-focusing direction of the ultrasonic probe 10 is D _NF and the distance x from the ultrasonic probe 10 is When the following formula (9) is satisfied, it can be represented by the following formula (10).

  Here, the focal point (x = F) in the focusing direction of the ultrasonic beam needs to satisfy the above formula (8), so the following formula (1) is imposed as a condition.

In this case, the beam cross-sectional area S ₂ of the beam focusing portion of the ultrasonic beam can be represented by the following formula (11).

On the other hand, regarding the inclusions, the length L of the inclusions is calculated using the following formula (6) and the formula (7) using the inclusion width d ₁ and the rolling ratio S ₀ / S _1. 12).

Then, as shown in FIG. 7A and FIG. 7B, considering the case where the unfocused direction of the ultrasonic beam and the extending direction of the inclusion are matched and the inclusion is at the center of the ultrasonic beam cross section, the ultrasonic beam sectional area S ₃ of the inclusions contained within can be represented by the following formula (13) and the following equation (14). In the following formula (13) and the following formula (14), when d _2NF <L, the cross section of the inclusion in the ultrasonic beam is approximated as a rectangle, and it is assumed that d _2F ≧ d ₁ . .

Next, consider S ₃ / S ₂ , which is the ratio of the beam cross-sectional area S ₂ of the ultrasonic beam and the cross-sectional area S ₃ of the inclusions included in the ultrasonic beam. Reflected signals A ₁ obtained from inclusion during ultrasonic flaw detection, the use of reflective signal A ₀ when the ultrasonic wave is reflected from the entire ultrasonic beam can be represented by the following formula (15). In the following formula (15), it is assumed that the reflectances per unit area in the reflected signals A ₁ and A ₀ are equal.

Here, since the ultrasonic flaw detection, it suffices to detect the reflected signals A ₁ corresponding to the inclusions detected size d ₃ or more inclusions set by inclusions detection size setting step described above, the ultrasonic inspection When the noise level is A _n and the margin for detection (SN ratio) is α, the reflected signal A ₁ is a value as shown in the following equation (16).

From the above, the conditions required for the beam width d _{2F in the} focusing direction and the beam width d _2NF in the non-focusing direction of the ultrasonic beam in order to detect inclusions where d ₁ ≧ d ₃ are the above formulas (11) to (11). Considering equation (16), the following equations (17) and (18) are obtained.

Here We think noise level _{A n} in formula (17) and the equation (18). We, the six different ultrasonic probe 10 specifications, as shown in FIG. 8, was compared reflected signal A ₀ and the noise level A _n actual equivalent conditions and the ultrasonic flaw detection . From each “reciprocal of S / N ratio” shown in the figure, it can be seen that A _n ≈0.01 · A ₀ suffices regardless of the specifications of the ultrasonic probe 10. Considering this, the above formula (17) and the above formula (18) become the following formula (19) and the following formula (20).

  The margin value α needs to satisfy at least α ≧ 2. In this case, the above formula (19) and the above formula (20) become the following formula (21) and the following formula (22).

  The margin value α is more preferably α ≧ 5. In this case, the above formula (19) and the above formula (20) become the following formula (23) and the following formula (24).

Further, with respect to the above formulas (21) and (22), when d _2F , d _2NF , and L are eliminated using the above formulas (8), (9), and (12), the following formulas (25) and (22) (26) And what added the said Formula (1) to following formula (25) and following formula (26) becomes a conditional expression in this step.

Similarly, when d _2F , d _2NF , and L are eliminated using the above formulas (8), (9), and (12) for the above formula (23) and the above formula (24), the following formula (27) and The following formula (28) is obtained. And what added the said Formula (1) to the following formula (27) and the following formula (28) becomes a more preferable conditional expression in this step.

  Based on the above, in this step, the ultrasonic probe 10 used in the ultrasonic flaw detection step to be described later satisfies the above formula (1), and the above formula (25) or the above formula (26) (preferably the above formula ( 27) or the above equation (28)). In other words, the ultrasonic probe 10 satisfies the above formula (1) and satisfies the following formula (2) when the following formula (2) is satisfied. It sets so that following formula (4) may be satisfy | filled.

Next, in the detection threshold setting step, an inclusion detection threshold _Ath is set. Here, since it is sufficient detect reflected signals _{A 1} inclusions corresponding to inclusions detection size _{d 3,} considering the above formula (11) to the equation (16), the detection threshold _{A th} is represented by the following formula (29 ) And the following formula (30).

Next, in the flaw detection pitch setting step, the flaw detection pitch p ₁ in the rolling direction of the steel material and the flaw detection pitch p ₂ in the direction perpendicular to the rolling direction of the steel material are determined. The flaw detection pitches p ₁ and p ₂ may be set based on the beam widths d _2F and d _2NF so that there is no flaw detection _defect . The flaw detection pitch p is set so as to satisfy the following formula (31) and the following formula (32) when considering the measurement points as shown in FIG.

  The steel material cleanliness evaluation method according to the present embodiment performs the evaluation condition setting step (step S1 in FIG. 5 and steps S11 to S14 in FIG. 6) as described above, and then the object evaluation step (step in FIG. 6). S2) is performed. In the subject evaluation step, as shown in FIG. 10, the subject preparation step (step S21), the ultrasonic flaw detection step (step S22), and the evaluation step (step S23) are performed in this order.

  First, in the subject preparation step, a subject for ultrasonic flaw detection is prepared. Specifically, in this step, for example, cutting of steel (round bar steel), surface smoothing, heat treatment for crystal grain refinement, and the like are performed. Further, in cutting out the steel material, the flaw detection surface is made parallel to the rolling direction as shown in FIG. Further, in order to efficiently perform water immersion two-dimensional flaw detection later, it is preferable to cut out the subject so that the flaw detection surface becomes a flat surface as shown in FIG.

Next, in the ultrasonic inspection step, ultrasonic inspection is performed. In this step, the water infiltration method is used to perform flaw detection with the line-focusing ultrasonic probe 10 with high accuracy and efficiency. In this step, the ultrasonic probe 10 set in the ultrasonic probe setting step is used to transmit and receive ultrasonic waves for each of the flaw detection pitches p ₁ and p ₂ set in the flaw detection pitch setting step. by for each position continue to detect the reflected signals a _1, generates two-dimensional map of the reflected signal level (the reflected signal intensity) of the (two-dimensional distribution).

  In this step, the flaw detection is performed while the unfocused direction of the ultrasonic beam is always parallel to the rolling direction of the steel material. Further, as a specific method of ultrasonic flaw detection in this step, when the flaw detection surface is a flat surface, it is preferable to use a C-scan flaw detection method as described in Reference 3, for example, as shown in FIG. . In the figure, the case of ultrasonic flaw detection using a point-focused ultrasonic beam is shown as an example, but the same applies to the case of line focusing.

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

  When the flaw detection surface is a curved surface, for example, as shown in FIG. 13, it is preferable to perform two-dimensional scanning by combining axial rotation and movement in a direction parallel to the axial direction.

  As described above, in this step, while the ultrasonic probe 10 that forms an ultrasonic beam is used to transmit and receive ultrasonic waves to the steel material that is the subject by water immersion, the scanning plane is parallel to the rolling direction. The steel material is scanned two-dimensionally.

Finally, in the evaluation step, the cleanliness of the steel material is evaluated based on the two-dimensional map of the reflected signal level. In this step, a two-dimensional map of the reflection signal level corresponding to the two-dimensionally scanned surface in the ultrasonic flaw detection step is acquired, and becomes equal to or higher than the signal level (detection threshold A _th ) corresponding to the inclusion detection size d _3. Evaluate the part. As a specific evaluation method in this step, for example, the number of detected inclusions is counted and evaluated, or the signal level of each detected inclusion is evaluated.

  According to the steel material cleanliness evaluation method and the cleanliness evaluation apparatus 1 according to the present invention described above, the conditions of ultrasonic flaw detection using line-focused ultrasonic waves are considered in consideration of stretching due to rolling of inclusions mixed during casting. Therefore, the inclusion detection ability can be sufficiently obtained. Also, by transmitting and receiving the ultrasonic beam so that the unfocused direction of the line-focused ultrasonic beam and the extension direction of the inclusions are parallel, the flaw detection pitch can be made coarse in the non-focused direction of the line-focused ultrasonic beam. Therefore, the number of measurement points per flaw detection area can be reduced and the evaluation efficiency can be improved.

Hereinafter, the present invention will be described more specifically with reference to examples. In this example, a numerical experiment assuming a case where a steel material rolled at a rolling ratio S ₀ / S ₁ = 100 is flawed by the method according to the present invention and the method using a conventional point-focused ultrasonic beam is used. went.

In the method according to the present invention, the inclusion detection size d ₃ = 20 μm, the ultrasonic probe has an ultrasonic frequency f = 50 MHz, the transducer width in the focusing direction is D _F = 3 mm, and the transducer in the non-focusing direction The width was set to D _NF = 6 mm and the underwater focal length F = 38 mm. Thereby, as shown below, the conditions of the above formula (1) and the above formula (4) in the ultrasonic probe setting step (or the conditions of the above formula (1) and the above formula (26)) are satisfied. Become. In this case, the beam width d _{2F in the} focusing direction is _0.37 mm, and the beam width d _{2NF in the} non-focusing direction is 6 mm.

In the ultrasonic flaw detection step in this example, the case where an inclusion having a diameter d ₀ = 0.5 mm at the time of casting is present in the flaw detection range was considered. In this case, the width d ₁ of the inclusion after stretching is 50 μm, and the length L of the inclusion after stretching is 50 mm. With respect to such inclusions, the results of flaw detection with the non-focusing direction aligned with the rolling direction by the line-focusing ultrasonic probe according to the present invention and conventional ultrasonic beam diameters of 0.37 mm and 6 mm, respectively, for comparison. FIG. 14 shows the result of flaw detection performed by the point-focusing ultrasonic probe according to the technology. In the figure, as a result of flaw detection, each reflected signal level and the number of measurement points per unit area necessary for flaw detection are shown.

  As shown in FIG. 14, when a point-focusing ultrasonic probe having an ultrasonic beam diameter of 0.37 mm is used, although the reflected signal level that can be detected is high, the number of measurement points increases, so that flaws cannot be detected efficiently. I understand. It can also be seen that when a point-focusing ultrasonic probe with an ultrasonic beam diameter of 6 mm is used, the signal level is low although the number of measurement points is small. On the other hand, when the line-focusing ultrasonic probe according to the present invention is used, the reflection signal level is high and the number of measurement points is small. Therefore, according to the present invention, it can be seen that both the detection capability of the reflected signal level and the efficiency of flaw detection can be achieved.

  The steel material cleanliness evaluation method and the cleanliness evaluation device according to the present invention have been specifically described above with reference to modes and examples for carrying out the invention. However, the gist of the present invention is limited to these descriptions. Rather, it should be construed broadly based on the claims. Needless to say, various changes and modifications based on these descriptions are also included in the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 Cleanliness evaluation apparatus 10 Line focusing ultrasonic probe 20 Control part 110 Point focusing ultrasonic probe

Claims (3)

A method for evaluating the cleanliness of a steel material, wherein the cleanliness of a steel material formed by rolling a cast steel slab is evaluated by ultrasonic flaw detection,
Wherein the ultrasonic sound wave inclusions in the in the steel material to be detected by the flaw, inclusions detected size setting step of setting a minimum length d ₃ in the rolling direction perpendicular to the width direction of the steel material,
The underwater focal length is F, the ultrasonic frequency is f, the underwater ultrasonic sound velocity is C, the transducer width in the focusing direction of the ultrasonic probe is D _F , and the transducer width in the non-focusing direction of the ultrasonic probe is D _NF , when the cross-sectional area in the direction perpendicular to the rolling direction of the steel slab is S ₀ and the cross-sectional area in the direction perpendicular to the rolling direction of the steel material is S ₁ , the line-focused ultrasonic beam used in the ultrasonic flaw detection When the ultrasonic probe for forming the electrode satisfies the following formula (1) and satisfies the following formula (2) when the following formula (2) is satisfied, the following formula (2) is not satisfied: Is an ultrasonic probe setting step for setting so as to satisfy the following formula (4);
Using the set ultrasonic probe, an ultrasonic beam is transmitted / received to / from the steel material by a water immersion flaw detection method so that the unfocused direction of the ultrasonic probe is parallel to the rolling direction of the steel material. Meanwhile, an ultrasonic flaw detection step for two-dimensionally scanning the steel material so that the scanning surface is parallel to the rolling direction;
An evaluation step of acquiring a two-dimensional distribution of ultrasonic reflection signal levels corresponding to the two-dimensionally scanned surface, and evaluating a portion having a signal level equal to or higher than the length d ₃ from the two-dimensional distribution;
A method for evaluating the cleanliness of a steel material, comprising:

The inclusions detection size setting step, cleanliness evaluation method of the steel according to claim 1, characterized in that setting the length d ₃ in 20μm or less. A steel material cleanliness evaluation apparatus for evaluating the cleanliness of a steel material formed by rolling a cast steel slab by ultrasonic flaw detection,
The underwater focal length is F, the ultrasonic frequency is f, the underwater ultrasonic sound velocity is C, the transducer width in the focusing direction of the ultrasonic probe is D _F , and the transducer width in the non-focusing direction of the ultrasonic probe is D _NF , when the cross-sectional area in the direction perpendicular to the rolling direction of the steel slab is S ₀ and the cross-sectional area in the direction perpendicular to the rolling direction of the steel material is S ₁ , the line-focused ultrasonic beam used in the ultrasonic flaw detection When the ultrasonic probe for forming the electrode satisfies the following formula (1) and satisfies the following formula (2) when the following formula (2) is satisfied, the following formula (2) is not satisfied: Is set to satisfy the following formula (4),
Using the set ultrasonic probe, an ultrasonic beam is transmitted / received to / from the steel material by a water immersion flaw detection method so that the unfocused direction of the ultrasonic probe is parallel to the rolling direction of the steel material. While scanning the steel material two-dimensionally so that the scanning surface is parallel to the rolling direction,
A two-dimensional distribution of ultrasonic reflection signal levels corresponding to the two-dimensionally scanned surface is acquired, and the rolling direction of the steel material of inclusions in the steel material to be detected by the ultrasonic flaw detection from the two-dimensional distribution minimum steel cleanliness evaluation apparatus characterized by evaluating a portion to be the signal level above which corresponds to the length d ₃ in the vertical width direction.

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