JP2004219086A - Cleanliness evaluation method of steel by immersion ultrasonic inspection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method allowing accurate evaluation of the cleanliness of a steel by carrying out fast detection and evaluation of non-metallic inclusions in response to the type of a steel product by using an immersion ultrasonic inspection method. <P>SOLUTION: The method is for evaluating the cleanliness of a steel, and it is provided with a process of sampling a test piece of steel which is an evaluation object from the forged steel product, a process of detecting the non-metallic inclusion existing in the test piece by using the immersion ultrasonic inspection method, and an inclusion evaluating process of evaluating the cleanliness of steel on the basis of a result obtained by the detecting process. In the inclusion evaluating process, the cleanliness of steel is evaluated by excluding a range of the following (1) or a range derived from an equation of the following (2) from an inclusion evaluation object range as a range where porosities are distributed in response to the carbon concentration of the test piece. (1) when the carbon concentration (C%) is ≤0.4%, the range (%) to be excluded from the center of the test piece is 30. (2) when the carbon concentration (C%) is >0.4%, the range (%) to be excluded from the center of the test piece is 27.3×(C%)+19.1. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
[Field of the Invention]
The present invention relates to a method for evaluating the cleanliness of steel by water immersion ultrasonic testing, and more particularly, to a method for evaluating steel cleanliness using water immersion ultrasonic testing (and without analyzing the phase or frequency of a defect wave). The present invention relates to a method for quickly assessing the level of nonmetallic inclusions present in steel, and for enabling rapid assessment of the cleanliness of steel.
[0002]
[Prior art]
In recent years, high-cleanliness steel has also been manufactured stably, and non-metallic inclusions in the small and medium diameter region in the steel have been further reduced (in this specification, “non-metallic inclusions” Is sometimes simply referred to as "inclusions").
[0003]
On the other hand, large (} area is 100μm or higher) oxide inclusions of (such as _{Al 2 O 3, MgO · Al} 2 O 3, CaO · Al 2 O 3 + MgO · Al 2 O 3) are still present For example, it causes fatigue fracture in steel materials such as bearing steel and carbon steel for machine structural use. However, such large inclusions are very difficult to detect because they appear with extremely low probability.
[0004]
Conventionally, as a method for inspecting inclusions in steel, a method of collecting a test piece from a steel material to be analyzed and inspecting the surface of the test piece with an optical microscope is a general method. "JIS G 0555 Microscopic test method for non-metallic inclusions in steel", "ASTM E45 Standard Practice for Determining the Inclusion Content of Steel", and the like. However, the inspection method using a microscope has a problem in that the detection area of a test piece is small, for example, 100 to 200 mm ² / piece, and thus the detection accuracy of large inclusions is extremely low.
[0005]
As an inspection method on the order of several tens to several hundreds of g, a method has been proposed in which inclusions are extracted from a steel material by acid dissolution and the particle size of the inclusions is evaluated with a microscope (for example,
[Patent Document 1],
See Patent Document 2). However, in the acid dissolution method, the inclusions may be dissolved in the acid or the inclusions may be dissolved to reduce the diameter of the inclusions. Attention must also be paid to disturbing substances. In addition, it takes a long time to dissolve the acid, so that the process is inferior in speed, and it is difficult to cope with a mass production process of a product.
[0006]
There is a slime method as an inspection method on the order of several kg. However, this method also lacks speed.
[0007]
On the other hand, as a method for inspecting inclusions in steel by ultrasonic testing, conventionally, two different types of foreign substances are discriminated by using simple phase information (P / A) of a defect wave (for example,
See Patent Document 3).
[0008]
In addition, by performing forging, the porosity in the steel material is pressed in advance, and then the cleanness is evaluated (for example,
See Patent Document 4).
[0009]
In addition, the defect type is determined based on the defect echo parameters (phase, center frequency, and intensity of the defect echo) and the parameters (bottom echo intensity, defect area) obtained from the waveform of the bottom reflection echo near the defect. (For example,
See Patent Document 5).
[0010]
Also, in ultrasonic flaw detection, two types of probes, a large probe and a small probe, are used, and defects detected by both probes are detected with porosity and only the large probe. The resulting defects are identified as non-metallic inclusions (for example,
See Patent Document 6).
[0011]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 9-125199 [Patent Document 2]
JP-A-9-125200 [Patent Document 3]
JP-A-8-75712 [Patent Document 4]
Japanese Patent Application Laid-Open No. Hei 9-17005 [Patent Document 5]
JP 2000-214142 A [Patent Document 6]
JP 2001-4602 A
[Problems to be solved by the invention]
However, in the above-mentioned prior art relating to ultrasonic flaw detection, there are the following problems.
[0013]
According to the method described in Patent Document 3, a probe having a long focal length ([focal length / vibrator diameter]) cannot be used, and a test piece having a predetermined thickness or more cannot be used. Waveform analysis by P / A cannot be performed. Also,
According to the method described in Patent Document 5 and the method described in Patent Document 6, it is difficult to distinguish large inclusions and small porosity. That is, the defect waveform information includes the phase, the defect wave intensity, and the center frequency. However, in the case of a focus type probe (for example, a transducer diameter of 1/2 inch and a focal length of 6 inches in water), the phase cannot be used because the phase forward rotation condition for the inclusion is not satisfied in principle. Further, depending on the defect wave intensity, it is difficult to distinguish between a large inclusion and a small porosity. Also,
According to the method described in Patent Document 4, especially in the vicinity of the center of the steel material, a large amount of porosity is present and cannot be easily removed. Therefore, irrespective of the type of the steel material, it is conceivable to evaluate the cleanliness of the steel material by uniformly excluding a predetermined range from the center. However, since the range from the center where the porosity exists differs depending on the type of steel, the cleanliness cannot be accurately evaluated depending on the type of steel depending on the method.
[0014]
An object of the present invention is to provide a method that enables rapid detection and evaluation of nonmetallic inclusions according to the type of steel material by using deep ultrasonic flaw detection, thereby enabling accurate evaluation of steel cleanliness. Is to do.
[0015]
[Means for Solving the Problems]
As a result of intensive studies, the present inventor has found that the range in which porosity exists near the center of a steel material forged at a predetermined ratio is determined by the concentration of carbon contained in the steel material (hereinafter referred to as “C%” as appropriate). I came to discover. Therefore, the range near the center where the porosity exists for each steel material with different carbon concentration was confirmed, and for the test piece with the carbon concentration within a predetermined range, a relational expression indicating the range where the porosity exists from the center of the test piece was derived. , A range to be excluded from the inclusion evaluation range.
[0016]
That is, the present invention is a method for evaluating the cleanliness of steel, a step of sampling a steel test piece to be evaluated from a steel material forged, and the test piece using a water immersion ultrasonic testing method. A step of detecting non-metallic inclusions present in the steel, and an inclusion evaluation step of evaluating the cleanliness of steel based on the result obtained in the detection step, comprising: According to the carbon concentration of the piece, the range of the following (1) or the range derived from the formula (2) is excluded from the inclusion evaluation range as the range of porosity distribution, and the steel cleanliness is evaluated. This is a method for evaluating the cleanliness of steel.
Carbon concentration (C%) ≦ 0.4% Range (%) excluded from the center of the test piece = 30 (1)
Carbon concentration (C%)> 0.4% Range (%) excluded from the center of the test piece = 27.3 × (C%) + 19.1 (2)
According to the method of the present invention, it is possible to appropriately exclude the porosity distribution range from the inclusion evaluation range for each steel material, and determine the cleanliness of the steel in the peripheral portion excluding the porosity distribution range based on the subjective opinion of the worker. It is possible to evaluate accurately without using.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention,%, which is a unit used when describing a range excluded from the inclusion evaluation range, refers to a test in which a ratio of a distance from a center point of a test piece to an outer periphery of the test piece is set to 100%. It represents the ratio of the distance from the center point of one piece. Further, the inclusions in the steel refer to, for example, oxides, nitrides, and sulfides. In addition, the water immersion ultrasonic inspection method uses water as a couplant, and in particular, the submerged ultrasonic inspection method involves submerging an entire test piece in water. The term “defect” refers to an inclusion or a cause such as porosity that causes a reflected wave other than the inclusion. Porosity refers to fine cavities generated inside metal during casting.
[0018]
Hereinafter, embodiments of the present invention will be described.
[0019]
First, a test piece is prepared. In preparing the test piece, a round bar is used, but the present invention is not limited to the round bar. Here, a billet prepared by forging a bloom is prepared. In general, in a metal material, as-cast, there are countless microporosity, and as described above, inspection may be difficult. On the other hand, a test piece taken from a billet obtained by forging a bloom with a forging ratio of 6 or more and 60 or less reduces the adverse effects such as irregular reflection of ultrasonic waves incident on the test piece.
[0020]
The test piece is obtained by cutting the above-mentioned billet into a plane perpendicular to the axial direction, and further cutting the billet into two substantially parallel planes separated at substantially equal intervals from the central axis. Thereafter, the surface A and the surface B are formed by milling, and then heat treatment is performed. As a result, the structure as-solidified, as-rolled, or as-forged is erased to obtain a fine and homogeneous structure, and the mechanical properties are improved.
[0021]
Finally, the surface A and the surface B are polished to make the surface of the test piece smooth. The test piece 1 shown in FIG. 5 created as described above has a small transmission loss on the ultrasonic wave incident surface, and enables accurate detection and evaluation of inclusions.
[0022]
Next, sensitivity calibration is performed. In such sensitivity calibration, for example, a flat bottom hole (Flat Bottom Hole (hereinafter, referred to as “FBH”)) having a diameter of 1.5 mm and a depth of 6 mm of the test piece is subjected to ultrasonic flaw detection to specify a position, and the φ1.5 FBH is determined. The ultrasonic flaw detector was set so that the maximum reflected wave intensity obtained when the ultrasonic wave was focused was 80%, at which time the sensitivity setting value was set as the reference sensitivity, and the sensitivity setting value sensitized by 18 dB from this was set. Was defined as the flaw detection sensitivity.
[0023]
Next, the test piece 1 manufactured as described above is subjected to ultrasonic testing by a water immersion ultrasonic testing method. In the method for evaluating the cleanliness of steel according to the present invention, ultrasonic flaw detection is performed. Various types of apparatuses for performing ultrasonic flaw detection are commercially available, and these can be used in the present invention. Preferred probes include a focus type probe. The detection capability of the flat probe is said to be 波長 wavelength, but the focus probe is １ ／ wavelength, which is difficult to detect. Evaluate inclusions with AREA of about 100 μm or more. Suitable to do.
[0024]
FIG. 7 illustrates a schematic diagram of an ultrasonic flaw detector using a focus type probe. A submerged water immersion ultrasonic flaw detector equipped with a focus type probe was used for ultrasonic flaw detection. The ultrasonic flaw detector comprises 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. The microprocessor incorporates an arithmetic processing program according to the flowchart shown in FIG. By providing such a PC in the ultrasonic flaw detector, a large amount of data processing can be rapidly performed.
[0025]
In performing ultrasonic flaw detection, the test piece 1 is set in a water tank, and then data of the test piece, measurement sensitivity, focus position, gate position, and flaw detection pitch are input to the PC 14. Then, the focus type probe 11 is operated to start ultrasonic flaw detection.
[0026]
The data input as described above is transmitted to the ultrasonic inspection unit 12 and the scanning unit 13, and the ultrasonic inspection is started under such conditions.
[0027]
That is, an ultrasonic wave is transmitted from the focused probe 11, hits the object, detects the reflected wave, and obtains the reflected wave intensity and reflected waveform information (waveform output as a graph, positive half wave intensity, negative half wave). Desired information based on the wave intensity). The scanning by the focus type probe 11 performs the emission of ultrasonic waves and the reception of reflected waves at a plurality of places at predetermined intervals on the test piece 1 (this interval is referred to as “flaw detection scanning pitch” or simply “scanning”). Pitch ”).
[0028]
Next, the cleanliness of the steel is evaluated based on the received reflected waveform information.
[0029]
The PC 14 is provided with a microprocessor in which a calculation program for detecting an inclusion is incorporated, so that a large amount of data can be quickly processed.
[0030]
Based on the data on inclusions in steel obtained as a result of such ultrasonic testing, the level of inclusions in steel (cleanness of steel material) can be evaluated. The data obtained here is the number, position, size, etc. of inclusions. For example, based on these data, the particle size distribution can be represented as a histogram to evaluate the cleanliness. In addition, data obtained by estimating the maximum inclusion diameter in the steel to be inspected can be obtained from the obtained actual measurement data by using, for example, a statistical method such as an extreme value statistical method.
[0031]
The evaluation of the cleanliness is performed by excluding a predetermined range from the center of the test piece. As described above, the cast slab is forged before ultrasonic testing. As a result, most of the porosity existing outside the predetermined range from the center is compressed and the range in which the porosity exists is narrowed. However, the porosity in a certain range from the center is still present without crimping.
[0032]
Therefore, by setting a range excluding a predetermined range in which porosity exists from the center of the test piece as the inclusion evaluation target range, it is possible to accurately evaluate the cleanliness of the steel material.
[0033]
The predetermined range where the porosity exists from the center of the test piece differs depending on the concentration of carbon contained in the steel material. Therefore, a predetermined range where porosity exists from the center of the test piece is clarified in advance, and a range to be excluded from the inclusion evaluation target range is determined. Such a range is determined as follows.
[0034]
First, the position where the porosity exists is confirmed by cutting away the test pieces having different carbon concentrations, and the result is plotted on the vertical axis in the radial direction where the porosity exists, that is, the porosity distribution range [% D] ([ % D], "D" is an abbreviation of DIAMETER and means diameter), and plotted on a graph in which the horizontal axis represents C%, which is the carbon concentration of the test piece.
[0035]
Then, when points indicating porosity present at positions farthest from the respective centers of test pieces having different carbon concentrations are connected, the graph shown in FIG. 1 is obtained.
[0036]
From this graph, the range to be excluded from the inclusion evaluation range can be determined as shown in the following (1) and (2) according to the carbon concentration of the test piece.
Carbon concentration (C%) ≦ 0.4% Range (%) excluded from the center of the test piece = 30 (1)
Carbon concentration (C%)> 0.4% Range (%) excluded from the center of the test piece = 27.3 × (C%) + 19.1 (2)
That is, for each steel type, the range of (1) or the range derived from the formula (2) becomes clear as the range to be excluded from the inclusion evaluation target range, and in the range excluding the range where the porosity exists, the steel material is accurately determined. Cleanliness can be measured. The method of the present invention is effective particularly for a material having a dangerous volume of about D / 4 from the surface, since the user can directly evaluate a steel material portion used directly as a material.
[0037]
The range of the test piece to be subjected to ultrasonic inspection is the gate portion 2 shown in FIG. 6, and the range 3 where porosity exists is excluded from the evaluation process after the scanning is completed. In addition, the dead zone 4, the outer peripheral portion 5, and the end portion 6 are excluded from the scanning range because they are so noisy that accurate detection of inclusions is difficult.
[0038]
The outer peripheral portion of the test piece is, for example, in a range of approximately 90 to 100% from the center.
[0039]
【Example】
Next, the method for evaluating the cleanliness of steel according to the present invention will be described in more detail with reference to examples. However, the method for evaluating the cleanliness of steel according to the present invention is not limited to the following examples.
[0040]
[Example 1]
A cut piece of SCM420 having a C% of 0.2% and a CC bloom of 380 mm × 490 mm which was forged at a forging ratio of 8.5 into a round bar steel having a diameter of 167 mm was prepared. Next, a test piece was cut out from the round bar into the shape shown in FIG. 5, formed by milling, then normalized at 900 ° C., and then the milled surface was polished to a flat surface, and 40 mm in the a direction and 40 mm in the b direction. It was formed to 167 mm and 80 mm in the c direction.
[0041]
The test piece was cut to confirm the presence of porosity, and the vertical axis represents the depth at which porosity exists, that is, the distribution range of porosity [% D], and the horizontal axis represents C%, which is the carbon concentration of the test piece. Plotted. Then, as shown in FIG. 2, the porosity located farthest from the center of the test piece was located at a position of approximately 29% D. This result is consistent with the range of the above (1), in which the range of 30% D from the center of the test piece is excluded from the inclusion evaluation range for the test piece having a carbon concentration of 0.4% or less. it is obvious.
[0042]
[Example 2]
A cut section of SCM435 having a C% of about 0.35% and a CC bloom of 380 mm × 490 mm prepared by forging a round bar steel having a diameter of 167 mm with a forging ratio of 8.5. Next, a test piece was cut out from the round bar into the shape shown in FIG. 5, formed by milling, then normalized at 870 ° C., and then the milled surface was polished to a flat surface, and 40 mm in the a direction and 40 mm in the b direction. It was formed to 167 mm and 80 mm in the c direction.
[0043]
The test piece was cut to confirm the presence of porosity, and the vertical axis represents the depth at which porosity exists, that is, the distribution range of porosity [% D], and the horizontal axis represents C%, which is the carbon concentration of the test piece. Plotted. Then, as shown in FIG. 3, the porosity located farthest from the center of the test piece existed at a depth of about 28% D. This result is consistent with the range of the above (1), in which the range of 30% D from the center of the test piece is excluded from the inclusion evaluation range for the test piece having a carbon concentration of 0.4% or less. it is obvious.
[0044]
[Example 3]
SUJ2 having a cut surface of approximately 0.95% was prepared by forging a CC bloom having a cut surface of 380 mm x 490 mm into a round bar steel having a diameter of 167 mm and a forging ratio of 8.5. Next, a test piece was cut out from the round steel bar into the shape shown in FIG. 5, formed by milling, then annealed at 800 ° C., and thereafter, the milled surface was polished to a flat surface, and 40 mm in the a direction and 40 mm in the b direction. It was formed to 167 mm and 80 mm in the c direction.
[0045]
The test piece was cut to confirm the presence of porosity, and the vertical axis represents the depth at which the porosity exists, that is, the distribution range of porosity [% D], and the horizontal axis represents C%, which is the carbon concentration of the test piece. Plotted. Then, as shown in FIG. 4, the porosity located farthest from the center of the test piece existed at a depth of about 45% D.
[0046]
When the conditions of this test piece are substituted into the above equation (2), it becomes 27.3 × 1.00 + 19.1 = 46.4%, and the result obtained by cutting corresponds to the result derived from the above equation (2). It is clear that.
[0047]
【The invention's effect】
As described above, according to the present invention, it is possible to quickly detect and evaluate nonmetallic inclusions according to the type of steel using the deep ultrasonic flaw detection method, and to accurately evaluate the cleanliness of steel. Can be provided.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between a maximum porosity distribution range and a carbon concentration.
FIG. 2 is a graph in which the distribution of porosity in a test piece of SCM420 is applied to the graph of FIG.
FIG. 3 is a graph in which the distribution of porosity in a test piece of SCM435 is applied to the graph of FIG.
FIG. 4 is a graph in which the porosity distribution in the test piece of SUJ2 is applied to the graph of FIG.
FIG. 5 is a perspective view showing a test piece prepared from a billet.
FIG. 6 is a perspective view showing a flaw detection surface of a test piece.
FIG. 7 is a schematic diagram showing an outline of an ultrasonic flaw detector.
FIG. 8 is a schematic diagram of an arithmetic processing program incorporated in a microprocessor.
[Explanation of symbols]
1 Test piece 2 Gate part 3 Center part 4 Dead zone 5 Outer part 6 End part 11 Focus type probe 12 Ultrasonic flaw detection unit 13 Scanning unit 14 ..PC
15 ・ ・ Visualization unit

Claims (1)

A method for evaluating the cleanliness of steel,
For a test piece of steel to be evaluated, a step of sampling from a pressed and forged steel material,
Detecting non-metallic inclusions present in the test piece using water immersion ultrasonic testing,
An inclusion evaluation step for evaluating the cleanliness of the steel based on the result obtained in the detection step,
In the inclusion evaluation step, depending on the carbon concentration of the test piece, a range derived from the following range (1) or (2) is excluded from the range of inclusion evaluation as a range of porosity distribution. A method for evaluating the cleanliness of steel, comprising evaluating the cleanliness of steel.
Carbon concentration (C%) ≦ 0.4% Range (%) excluded from the center of the test piece = 30 (1)
Carbon concentration (C%)> 0.4% Range (%) excluded from the center of the test piece = 27.3 × (C%) + 19.1 (1)

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