JP2017156273A - Cleanliness evaluation method for forged-steel product - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cleanliness evaluation method capable of evaluating cleanliness of a forged-steel product with relatively high accuracy.SOLUTION: The present invention provides a method for evaluating cleanliness of a specific evaluation object surface of a forged-steel product. The method includes a step of measuring a maximum non-metallic inclusion dimension aof n-number of inspection surfaces, a step of converting the maximum non-metallic inclusion dimension aof each inspection surface into a projection dimension a'at an evaluation surface by using an angle θof each of the n-number of inspection surfaces in a forged direction and an angle θ' of the evaluation surface in the forged direction, and a step of calculating, from the projection dimension a'at the evaluation surface, an estimated maximum non-metallic inclusion dimension aat the evaluation surface by using statistics of extreme value. The conversion is preferably performed on an assumption that the non-metallic inclusion is a spheroid having a long axis in the forged direction. The converting step preferably includes a step of primary converting the maximum non-metallic inclusion dimension ainto a primary projection dimension a''with respect to a reference surface having an angle of 0° relative to the forged direction, and a step of secondary converting the primary projection dimension a''into the projection dimension a'of the evaluation surface.SELECTED DRAWING: None

Description

  The present invention relates to a method for evaluating the cleanliness of forged steel products.

  High fatigue strength is required for mechanical structural members that use forged steel products and the like. In such a mechanical structural member, non-metallic inclusions present in the metal material are likely to be the starting point of fatigue failure. Therefore, in order to improve the fatigue strength of the mechanical structural member, there is a demand for an accurate evaluation technique for non-metallic inclusions as well as a technique for reducing and reducing non-metallic inclusions in the metal material.

  On the other hand, the recent progress in steelmaking technology has greatly improved the cleanliness of metal materials, and the probability of large non-metallic inclusions affecting fatigue strength has become very low. Inclusion detection is very difficult. Here, since the cleanliness of the metal material is determined by the size of the nonmetallic inclusions, it is difficult to evaluate the cleanliness of the metal material because it is difficult to detect the nonmetallic inclusions. Yes.

  As described above, the extreme statistical method is used as a method for evaluating the cleanliness of the metal material even when the probability of occurrence of large non-metallic inclusions is low, in contrast to the difficulty in evaluating the cleanliness of the metal material. Methods have been proposed (see, for example, Japanese Patent Application Laid-Open Nos. 2001-141704, 2000-214142, and 2012-73059).

In the method using the extreme value statistical method proposed in the above publication, first, n inspection parts are set for the metal material to be inspected, non-metallic inclusions are measured for each inspection part, and each inspection part is measured. Find the maximum non-metallic inclusion size. Next, the method predicts the maximum non-metallic inclusion size present in the metallic material based on each maximum non-metallic inclusion size determined at n inspection sites. Specifically, the maximum non-metallic inclusion dimensions a _j (j = 1, 2, 3,..., N) and the normalization variable y _j = −ln [−ln] arranged in ascending order at n inspection sites. From the linear regression equation created from {j / (n + 1)}] and the value of y _max , the maximum non-metallic inclusion size existing in the metal material is predicted. In addition, the measurement of the nonmetallic inclusion in each said test | inspection site | part is performed by an ultrasonic flaw, an ultrasonic fatigue test, etc.

  However, in the forged steel product, the shape of the non-metallic inclusions is deformed into a shape extended in one direction by forging, and therefore the inclusion size measured by the direction of the inspection surface changes even for the same inclusion. Therefore, in the cleanliness evaluation of a forged steel product, depending on how the inspection surface is taken, the predicted inclusion size may deviate greatly from the inclusion size on the evaluation surface where the cleanliness was actually evaluated.

JP 2001-141704 A JP 2000-214142 A JP 2012-73059 A

  This invention is made | formed based on the above situations, and it aims at providing the cleanliness evaluation method which can evaluate the cleanliness of a forged steel product with comparatively high precision.

The invention made in order to solve the above-mentioned problem is a cleanliness evaluation method on a specific evaluation surface in a forged steel product, and is a maximum non-metallic inclusion size a _i (i = 1 to n) on n inspection surfaces. And the angle θ _i (i = 1 to n) with respect to the forging direction of the n inspection surfaces and the angle θ ′ with respect to the forging direction of the evaluation surface, and the maximum nonmetal of each inspection surface Using the process of converting the inclusion size a _i to the projected size a ′ _i (i = 1 to n) on the evaluation surface and the extreme value statistical method, the estimated maximum on the evaluation surface from the evaluation surface projected size a ′ _i And a step of calculating a non-metallic inclusion dimension a _max .

The forged steel product cleanliness evaluation method uses the maximum non-metallic inclusion dimension a _i obtained from a plurality of inspection surfaces, the angle θ _{i of} these inspection surfaces with respect to the forging direction, and the forging of the evaluation surface on which the cleanliness is to be evaluated. The angle θ ′ with respect to the drawing direction is used to convert the projection dimension a ′ _i onto an arbitrary evaluation surface inside the forged steel product. The evaluation plane projection dimension a ′ _i is obtained by replacing the dimension a _i of the maximum nonmetallic inclusion measured on the plurality of inspection planes with the dimension of the nonmetallic inclusion that can be measured on the evaluation plane. cleanliness evaluation method of the forgings, by calculating the estimated maximum non-metallic inclusion size a _max from the evaluation surface projection size a _'i, be evaluated with relatively high accuracy the cleanliness of the evaluation surface it can.

  The conversion may be performed on the assumption that the nonmetallic inclusion is a spheroid whose major axis is the forging direction. In general, nonmetallic inclusions in a metal material are spherical, and in a forged steel product, the nonmetallic inclusions become a spheroid whose major axis is the forging direction due to forging. Therefore, by making the above assumption and converting the dimensions of the nonmetallic inclusions, the evaluation procedure can be simplified while maintaining the accuracy.

The converting step primarily converts the maximum non-metallic inclusion size a _i into a primary projection size a ″ _i on a reference plane whose angle with respect to the forging direction is 0 °; and the primary projection size a ″ _i the may have a step of converting the secondary to the evaluation surface projection size a _'i. By converting the maximum non-metallic inclusion size a _i to the evaluation surface projection size a ′ _i by such a procedure, the conversion to the evaluation surface projection size a ′ _i can be easily and reliably performed.

In the primary conversion step, the maximum non-metallic inclusion size a _i may be converted to the primary projection size a ″ _i using the following formula (1). Thus, the primary projection dimension a ″ _i can be obtained relatively easily by performing the primary conversion using the known non-metallic inclusion aspect ratio γ on the reference plane.
a ″ _i = a _i · (γ / (sin ² θ _i + γ ² · cos ² θ _i ) ^1/2 ) ^1/2 (1)
In the formula (1), γ is an aspect ratio of nonmetallic inclusions on the reference plane.

The secondary transformation step uses the following formula (2) and formula (3), and the major axis l _i and minor axis of the non-metallic inclusions having an aspect ratio γ projected from the primary projection dimension a ″ _i onto the reference plane: Using the step of calculating s _i , the following formula (4), formula (5), and the aspect ratio γ ′ of the non-metallic inclusions in the evaluation region, the aspect ratio γ ′ is calculated from the major axis l _i and the minor axis s _i. Using the step of calculating the major axis l ′ _i and minor axis s ′ _i of the metal inclusions and the following formulas (6) and (7), projection from the major axis l ′ _i and minor axis s ′ _i onto the evaluation surface is performed. 'using a step of calculating a _i, the following expression (8), the major axis l' and _'i and the minor axis s'' diameter l of non-metallic inclusions' aspect ratio γ was' _i and the minor axis s'_'i And calculating the evaluation surface projection dimension a ′ _i from the above. By performing the secondary conversion in this way, the evaluation surface projection dimension a ′ _i can be obtained relatively easily from the primary projection dimension a ″ _i .
a ″ _i = (l _i · s _i · π) ^1/2 (2)
γ = l _i / ' _i (3)
l ′ _i = (s _i ² · l _i ) ^1/3 · (γ ′) ^2/3 (4)
s ′ _i = ((s _i ² · l _i ) / γ ′) ^1/3 (5)
l ″ _i = (s _i ² · sin ² θ ′ + (γ ′) ² · cos ² θ ′) ^1/2 (6)
s ″ _i = s ′ _i (7)
a ′ _i = (l ″ _i · s ″ _i · π) ^1/2 (8)

Another invention made in order to solve the above-mentioned problem is a cleanliness evaluation method on a specific evaluation surface in a forged steel product, and the maximum non-metallic inclusion size on n inspection surfaces having the same angle to the forging direction a step of measuring a _i (i = 1 to n), a step of calculating an estimated maximum non-metallic inclusion size a _max on the inspection surface from the maximum non-metallic inclusion size a _i using an extreme value statistical method; , Using the angle θ relative to the forging direction of the inspection surface and the angle θ ′ relative to the forging direction of the evaluation surface, the estimated maximum nonmetallic inclusion size a _max on the inspection surface is estimated to be the estimated maximum nonmetallic inclusion on the evaluation surface. And a process for converting to dimension a ′ _max .

The method for evaluating the cleanliness of the forged steel product uses the angle to the forging direction of each of the inspection surface and the evaluation surface, and the estimated maximum nonmetallic inclusion obtained from the dimension a _i of the maximum nonmetallic inclusion measured on a plurality of inspection surfaces. The object dimension a _max is converted into an estimated maximum non-metal inclusion dimension a ′ _max of non-metallic inclusions that can be measured on the evaluation surface. Therefore, the cleanliness evaluation method of the forged steel product can evaluate the cleanliness on the evaluation surface with relatively high accuracy.

The term “non-metallic inclusions” refers to non-metallic inclusions that are precipitated or entrained in the metallic material during the solidification process of the metallic material. For example, sulfide-based materials such as manganese sulfide (MnS), aluminum oxide ( These are inclusions such as oxides such as Al ₂ O ₃ ) and silicon dioxide (SiO ₂ ), and nitrides such as titanium nitride (TiN). “Non-metallic inclusion size” means the size represented by the projected area of the non-metallic inclusion on the inspection or evaluation surface. In addition to this projected area, for example, the true area of the projected area is equal to the projected area. The diameter of the circle, the half power value (√area) of the projected area, and the like are included.

  As described above, according to the method for evaluating the cleanliness of a forged steel product according to the present invention, the cleanliness of the forged steel product can be evaluated with relatively high accuracy.

It is an example of observation of non-metallic inclusions on an inspection surface (left side) having an angle of 90 ° with respect to the forging direction and an inspection surface (right side) having the angle of 0 °. It is a schematic diagram which shows the relationship between the angle with respect to a test | inspection surface and a forge direction, and the nonmetallic inclusion dimension measured. It is a schematic diagram which shows the forged steel product used in the Example.

  Hereinafter, the embodiment of the cleanliness evaluation method of a forged steel product concerning the present invention is described.

[First embodiment]
The forged steel product cleanliness evaluation method is a method for evaluating the cleanliness of a specific evaluation surface different from the inspection surface in the forged steel product. The method for evaluating the cleanliness of the forged steel product includes a step of measuring the maximum non-metallic inclusion size a _i (i = 1 to n) on n inspection surfaces (measurement step), and forging of the n inspection surfaces. Using each angle θ _i (i = 1 to n) with respect to the direction and the angle θ ′ with respect to the forging direction of the evaluation surface, the maximum non-metallic inclusion size a _i of each inspection surface is the projected dimension a ′ _i on the evaluation surface. (I = 1 to n) step of converting (converting step) and step of calculating the estimated maximum non-metallic inclusion size a _max on the evaluation surface from the evaluation surface projection size a ′ _i using the extreme value statistical method (Calculation step).

[Measurement process]
In the measurement process, n inspection surfaces are set on the forged steel product to be inspected, and the size of the nonmetallic inclusions on each inspection surface is measured.

Each angle θ _i (i = 1 to n) with respect to the forging direction of each inspection surface may be the same, but it is preferable that different angles are included from the viewpoint of evaluation accuracy. The angle theta _i is an angle with respect to forging direction in the central longitudinal plane of the forgings, is less than 90 ° 0 ° or more. The forging direction is the direction of the forging line of the steel material formed by forging, and the angle θ _i is measured or determined by observing the forging line by cross-sectional macro observation of the forged steel product. Can do. Also, if the shape and forging conditions are the same, the direction of the forging line will be almost the same, so by collecting the macro observation results with a charge (forged steel product) that is different from the object of cleanliness evaluation, Measurement of direction can be omitted.

  As a metal material forming the forged steel product, iron (Fe), aluminum (Al), titanium (Ti), nickel (Ni), copper (Cu), zinc (Zn), silver (Ag), gold (Au), These alloys, a magnesium (Mg) alloy, a chromium (Cr) alloy, etc. are mentioned.

  The measurement method in the measurement step is not particularly limited as long as it is a method capable of measuring nonmetallic inclusions in a metal material. Measurement method by ultrasonic flaw detection, measurement method by ultrasonic fatigue test, measurement method by electron microscope observation Etc. can be used. Among these, measurement by ultrasonic flaw detection is preferable because it is basically a nondestructive inspection method and can be inspected quickly.

  In the measurement step, arbitrary n inspection surfaces for measuring nonmetallic inclusions are set in the inspection area of the forged steel product. Here, the measurement in the measurement step may be performed by cutting out the inspection region from the metal material, or may be performed on the region of the inspection surface set without cutting out from the metal material.

  The number n of inspection surfaces set in the forged steel product is preferably 5 or more and 100 or less in statistical calculation.

  When measuring by cutting out the inspection surface, it is preferable to cut out a plurality of inspection samples having a rectangular parallelepiped shape of the same size including the inspection surface. By cutting out the inspection sample having the same shape in this way, continuous inspection and automatic measurement are facilitated.

The area of the inspection surface is preferably 1 mm ² or more and 10,000 mm ² or less. If the inspection area is less than the lower limit, the detection accuracy of non-metallic inclusions on the inspection surface may be reduced. Conversely, if the inspection area exceeds the upper limit, it may be difficult to measure nonmetallic inclusions on the inspection surface.

  Moreover, it is preferable to set a some test | inspection surface in each of the outer peripheral part and center part of the forged steel product to be examined. This is because, in steel materials, the central part is generally the final solidification position, and the discharge of nonmetallic inclusions into concentrated molten steel and the amount of nonmetallic inclusions settled are large, so the central part and the outer peripheral part are inspected in this way. This is because the detection rate of large non-metallic inclusions can be improved. As a result, the accuracy of cleanliness evaluation can be further improved.

  In addition, since the cast metal material generally has an infinite number of fine cavities, when scanning by the ultrasonic flaw detection method, an infinite number of irregular reflections and noises are generated by the cavities, and the measurement may not be performed normally. Therefore, when the cast metal material is an evaluation object and the ultrasonic flaw detection method is used in the measurement process, it is preferable to roll or forge the metal material including the inspection surface before measuring the inspection surface. By rolling or forging the metal material to be inspected in this way, the cavity disappears due to the pressure bonding of the metal material, so that the normal measurement of the inspection region by ultrasonic flaw detection can be performed.

[Conversion process]
In the conversion step, the maximum non-metallic inclusion size of each inspection surface obtained in the measurement step using each angle θ _{i with} respect to the forging direction of the n inspection surfaces and the angle θ ′ with respect to the forging direction of the evaluation surface. a _i is converted into a projected dimension a ′ _i (i = 1 to n) on the evaluation surface.

As shown in FIGS. 1 and 2, even when the same metal inclusion S is measured, the measured inclusion size changes depending on the orientation of the inspection surface I with respect to the forging direction P. Therefore, in this conversion step, the maximum non-metallic inclusion size a _i measured on the inspection surface is converted into the projected size a ′ _i on the evaluation surface.

Angle theta 'relative to forging direction of the evaluation plane, like the angle theta _i, is the angle relative to forging direction in the central longitudinal plane of the forgings, is less than 0 ° or 90 °. Moreover, angle (theta) 'can be measured by observing a forge stream line by cross-sectional macro observation of a forged steel product.

  Specifically, the conversion step can be easily performed by assuming that the nonmetallic inclusion is a spheroid whose major axis is the forging direction.

Further, the conversion step includes a step of converting the maximum non-metallic inclusion size a _i into a primary projection size a ″ _i on a reference plane whose angle with respect to the forging direction is 0 ° (primary conversion step), A step (secondary conversion step) of secondarily converting the primary projection size a ″ _i to the evaluation surface projection size a ′ _i .

  Here, since the degree of deformation due to forging of nonmetallic inclusions varies depending on the region in the forged steel product, the aspect ratio γ of the nonmetallic inclusions in the reference surface and the aspect ratio γ ′ of the nonmetallic inclusions in the evaluation region By using it, evaluation accuracy can be increased.

  The aspect ratio of the non-metallic inclusions on the reference surface and the evaluation surface can be obtained, for example, by observing the fatigue test fracture surface in each region. Further, the aspect ratio of the nonmetallic inclusions on the reference surface may be calculated from the aspect ratio of the nonmetallic inclusions on the inspection surface obtained from the above observation.

<Primary conversion process>
In the primary conversion step, the maximum non-metallic inclusion size a _i is converted into the primary projection size a ″ _i using the following formula (1).
a ″ _i = a _i · (γ / (sin ² θ _i + γ ² · cos ² θ _i ) ^1/2 ) ^1/2 (1)

<Secondary conversion process>
The secondary conversion step includes a step (first calculation step) of calculating a major axis l _i and a minor axis s _i of a non-metallic inclusion having an aspect ratio γ projected on a reference plane, and a non-metallic inclusion having an aspect ratio γ ′. a step (second calculation step) for calculating the major axis l _'i and the minor axis s' _i, a' _i and the minor axis s'_'i' major axis l of non-metallic inclusions' aspect ratio γ projected onto the evaluation plane A step of calculating (third calculation step) and a step of calculating the evaluation surface projection dimension a ′ _i (fourth calculation step).

(First calculation process)
In the first calculation step, the following formulas (2) and (3) are used, and the major axis l _i and the minor axis l _i of the non-metallic inclusion having the aspect ratio γ ″ projected from the primary projection dimension a ″ _i onto the reference plane are used. The diameter s _i is calculated.
a ″ _i = (l _i · s _i · π) ^1/2 (2)
γ = l _i / s _i (3)

(Second calculation step)
In the second calculation step, the following formula (4), formula (5) and the aspect ratio γ ′ of the nonmetallic inclusion in the evaluation region are used, and the nonmetallic inclusion having the aspect ratio γ ′ from the major axis l _i and the minor axis s _i is used. The major axis l ′ _i and the minor axis s ′ _i of the object are calculated.
l ′ _i = (s _i ² · l _i ) ^1/3 · (γ ′) ^2/3 (4)
s ′ _i = ((s _i ² · l _i ) / γ ′) ^1/3 (5)

(Third calculation step)
In the third calculation step, using the following formula (6) and formula (7), the major axis l ′ of the non-metallic inclusion having the aspect ratio γ ′ projected from the major axis l ′ _i and the minor axis s ′ _i onto the evaluation surface. ' _i and minor axis s'' _i are calculated.
l ″ _i = (s _i ² · sin ² θ ′ + (γ ′) ² · cos ² θ ′) ^1/2 (6)
s ″ _i = s ′ _i (7)

(Fourth calculation step)
In the fourth calculating step, using the following equation (8), calculates the evaluation surface projection size a _'i from the major axis l'_'i and the minor axis s'' _i.
a ′ _i = (l ″ _i · s ″ _i · π) ^1/2 (8)

[Calculation process]
In the calculation step, an estimated maximum non-metallic inclusion size a _max on the evaluation surface is calculated from the evaluation surface projection size a ′ _i obtained in the conversion step using an extreme value statistical method.

As the extreme value statistical method, for example, the following method can be used. First, the evaluation surface projection dimensions a ′ _i are arranged in ascending order, and the evaluation surface projection dimensions a ′ _j (j = 1 to n) arranged in this ascending order, and the standardization variable y _j = −ln [−ln {j / ( n + 1)}], an m-th order regression equation of the following formula (9) is derived with y _j as a dependent variable and a _j as an independent variable. The m-th order regression equation can be derived by using a known method such as a least square method or a maximum likelihood method, which can derive a first-order or higher-order regression equation.
y _j = f _m (a _j ) (9)

The solution of the m-th order regression equation of the above equation (9) obtained by the above derivation and the following equation (10) is obtained, and the estimated maximum nonmetallic inclusion size a _max on the evaluation surface is calculated.
f _m (a _max ) = y _max (10)

In the above equation (9), f _m (a _j ) can be, for example, a _j + b (b is a constant). In equation (10), y _max is, for example, ln [−ln {S _d / (S _d + S _e ). }]. S _d is the surface area of stress loading in the evaluation area is added (hazardous area), S _e is the surface area (hazardous area) stress load in the examination region is applied.

In this calculation step, it is preferable to calculate the maximum non-metallic inclusion size a _max after removing abnormal values from the evaluation surface projection size a ′ _i . This is because in the measurement step, things other than non-metallic inclusions may be determined as non-metallic inclusions and measured as non-metallic inclusion dimensions. For example, in the measurement by ultrasonic flaw detection, a reflected wave from a cavity that is not a non-metallic inclusion or an irregular reflection noise from outside may be measured as a non-metallic inclusion. On the other hand, data from defects that are not non-metallic inclusions can be omitted by removing abnormal values. For example, in ultrasonic flaw detection, since the waveform of an abnormal value and a normal value are different, the abnormal value can be easily removed by the waveform.

Based on the estimated maximum non-metallic inclusion size a _max obtained in this calculation step, the cleanliness of the forged steel product in the evaluation target region can be evaluated.

[advantage]
The forged steel product cleanliness evaluation method uses the maximum non-metallic inclusion dimension a _i obtained from a plurality of inspection surfaces, the angle θ _{i of} these inspection surfaces with respect to the forging direction, and the forging of the evaluation surface on which the cleanliness is to be evaluated. Since the angle θ ′ with respect to the stretching direction is used to convert the projected dimension a ′ _i onto the evaluation surface, the cleanliness on the evaluation surface can be evaluated with relatively high accuracy from the measurement value on the inspection surface.

[Second Embodiment]
The method for evaluating the cleanliness of the forged steel product is a method for evaluating the cleanliness of a specific evaluation surface in the forged steel product. The cleanliness evaluation method of the forged steel product includes a step (measurement step) of measuring the maximum non-metallic inclusion dimension a _i (i = 1 to n) on n inspection surfaces having the same angle with respect to the forging direction, and an extreme value. Using a statistical method, a step (calculation step) of calculating an estimated maximum non-metallic inclusion size a _max on the inspection surface from the maximum non-metallic inclusion size a _i , an angle θ with respect to the forging direction of the inspection surface, and the above 'using the estimated maximum non-metallic inclusion size a _max of the inspection surface estimated maximum non-metallic inclusion size a of the evaluation plane' angle θ with respect to forging direction evaluation plane step of converting the _max and (conversion step) Is provided.

[Measurement process]
The said measurement process is the same as the measurement process of the cleanliness evaluation method of the forged steel product of 1st embodiment except the point which makes the angle with respect to the forge direction of n inspection surface the same.

[Calculation process]
The calculation step uses an extreme value statistical method to calculate the estimated maximum non-metallic inclusion size a _max on the inspection surface from the maximum non-metallic inclusion size a _i obtained in the measurement step. The extreme value statistical method described in the first embodiment can be used.

[Conversion process]
The conversion step uses the angle θ with respect to the forging direction of the inspection surface and the angle θ ′ with respect to the forging direction of the evaluation surface, and evaluates the estimated maximum non-metallic inclusion size a _max on the inspection surface obtained in the calculation step. Convert to the estimated maximum non-metallic inclusion size a ′ _max at the surface. As a method of converting the estimated maximum non-metallic inclusion size a _max in the inspection surface on the estimated maximum non-metallic inclusion size a _'max of the evaluation plane, the conversion step of forgings cleanliness evaluation method of the first embodiment The method of converting the maximum non-metallic inclusion dimension a _i described in the above into the projected dimension a ′ _i on the evaluation surface can be used.

[advantage]
The method for evaluating the cleanliness of the forged steel product uses the angle to the forging direction of each of the inspection surface and the evaluation surface, and the estimated maximum nonmetallic inclusion obtained from the dimension a _i of the maximum nonmetallic inclusion measured on a plurality of inspection surfaces. The object dimension a _max is converted into an estimated maximum non-metal inclusion dimension a ′ _max of non-metallic inclusions that can be measured on the evaluation surface. Therefore, the cleanliness evaluation method of the forged steel product can evaluate the cleanliness on the evaluation surface with relatively high accuracy.

[Other Embodiments]
The cleanliness evaluation method of the present invention is not limited to the above embodiment.

That is, in the cleanliness evaluation method of the forged steel product, the method for converting the maximum non-metallic inclusion dimension a _i on the inspection surface into the projected dimension a ′ _i on the evaluation surface is not limited to the above embodiment, The conversion may be performed using a procedure. For example, the maximum non-metallic inclusion size a _i may be directly converted into the projected dimension a ′ _i on the evaluation surface without going through the primary projected dimension a ″ _i .

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

  First, as a forged steel product to be evaluated, a shaft forged steel product having a diameter of 350 mm was made. This shaft-based forged steel product is machined so that the center axis of the shaft is offset (eccentric) from the end B by forging from one direction in the radial direction as shown in FIG. This central part A was used as an evaluation area. Further, due to this training, in the central portion A, the forging direction (direction of the forging line) P changes along the axial direction as shown in the figure.

Next, a total of 12 small fatigue test pieces were collected from the end portion B of the shaft-based forged steel product, each having 6 small fatigue test pieces that were subjected to fatigue fracture on the surface where the angles with respect to the forging direction P were 0 ° and 90 °. Further, from the central portion A, one large fatigue test piece that was subjected to fatigue fracture on a surface having an angle with respect to the forging direction P of 60 ° was collected. These test pieces were sampled so that the fracture start site was a site that reached the 1 / 3R position of the steel ingot before forging. Further, with respect to these test pieces, a dangerous region where 90% of the maximum load stress was applied from the maximum load stress in the fatigue test was defined as an inspection region or an evaluation region. As a result of the fatigue test, the surface area of the inspection region of the small fatigue test piece was 676 mm ² , and the surface area of the evaluation region of the large fatigue test piece was 3320 mm ² .

  The fatigue test fracture surface was observed for the small fatigue test piece and the large fatigue test piece, and the maximum non-metallic inclusion size was determined. Table 1 shows the results of arranging the maximum non-metallic inclusion dimensions in this small fatigue test piece in ascending order.

  On the other hand, the maximum inclusion size on the evaluation surface (fatigue fracture surface) of the large fatigue test piece was 98 μm, and the aspect ratio γ ′ was 14.1.

Furthermore, when the fatigue test fracture surface was observed for another small fatigue test piece that was fatigue fractured at a surface where the angle with respect to the forging direction P was 75 ° (θ ₁ ), the average aspect γ ₁ of the nonmetallic inclusions was 3. .15. When this aspect ratio γ ₁ was converted into the aspect ratio γ of the reference surface having an angle with respect to the forging direction P of 0 ° using the following formula (11), 3.92 was obtained. Note that the aspect ratio γ of the reference surface may be directly determined by observation using a test piece having an inspection surface (fatigue fracture surface) whose angle with respect to the forging steel direction P is 0 °.
γ = ((γ ₁ ² −sin ² θ ₁ ) / cos ² θ ₁ ) ^1/2 (11)

The maximum projected non-metallic inclusion size of a small fatigue test piece having an inspection surface (fatigue fracture surface) with an angle with respect to the forging / stretching direction P of 90 ° is the primary projected dimension a on the reference surface using the above equation (1). into a '' _i, further the above formula (2), Table 2 shows the result of calculating the major axis l _i and the minor axis s _i (3) using a non-metallic inclusions of an aspect ratio gamma.

Furthermore, the results of calculating the major axis l ′ _i and minor axis s ′ _i of the nonmetallic inclusion having the aspect ratio γ ′ from the major axis l _i and the minor axis s _i using the above formulas (4) and (5) are shown. 3 shows.

Further, the formula (6), calculates the _'i and the minor axis s'' _i 'major axis l of non-metallic inclusions' (7) with a major axis l' _i and the minor axis s' aspect ratio from _i gamma Table 4 shows the results.

Furthermore, using the above equation (8) to calculate the evaluation surface projection size a _'i from the major axis l'_'i and the minor axis s'' _i. The results are shown in Table 5 together with the normalization variable y _j = −ln [−ln {j / (n + 1)}].

Finally, with respect to the obtained evaluation surface projection dimension a ′ _i , the approximate expression of the standardization variable is expressed as y _j = a _j + b (b is a constant), y _max = ln [−ln {S _d / (S _d + S _e )}] was used to calculate the estimated maximum non-metallic inclusion size a _max on the evaluation surface, and 99.8 μm was obtained as a _max . Thereby, it turns out that evaluation close | similar to measured value 98 micrometers can be performed by this invention. On the other hand, when the extreme value statistical method is used using the maximum non-metallic inclusion size in Table 1, the estimated maximum non-metallic inclusion size is obtained using only the result of the inspection surface where the angle with respect to the forging direction P is 0 °. Is 129.1 μm, and using only the result of the inspection surface where the angle is 90 °, the estimated maximum non-metallic inclusion size is 37.4 μm, which is greatly different from the actually measured value.

  As described above, since the cleanliness evaluation method can evaluate the cleanliness of the forged steel product with relatively high accuracy, it can guarantee the cleanliness of the forged steel product that requires high fatigue strength such as a crankshaft.

A Center B End I Inspection surface P Forging direction S Non-metallic inclusions

Claims (6)

A cleanliness evaluation method for a specific evaluation surface in a forged steel product,
measuring a maximum non-metallic inclusion dimension a _i (i = 1 to n) on n inspection surfaces;
Using each angle θ _i (i = 1 to n) with respect to the forging direction of the n inspection surfaces and the angle θ ′ with respect to the forging direction of the evaluation surface, the maximum nonmetallic inclusion size a _i of each inspection surface is obtained. Converting the projected dimension a ′ _i (i = 1 to n) on the evaluation surface;
A method for evaluating the cleanliness of a forged steel product, comprising: using an extreme value statistical method and calculating an estimated maximum nonmetallic inclusion size a _max on the evaluation surface from the evaluation surface projection size a ′ _i .   The method for evaluating the cleanliness of a forged steel product according to claim 1, wherein the conversion is performed on the assumption that the nonmetallic inclusion is a spheroid whose major axis is the forging direction. The conversion step is
Primary conversion of the maximum non-metallic inclusion dimension a _i into a primary projection dimension a ″ _i on a reference plane having an angle with respect to the forging direction of 0 °;
The method for evaluating the cleanliness of a forged steel product according to claim 2, further comprising a step of secondarily converting the primary projection dimension a ″ _i to the evaluation plane projection dimension a ′ _i . The method for evaluating the cleanliness of a forged steel product according to claim 3, wherein, in the primary conversion step, the maximum non-metallic inclusion size a _i is converted into the primary projected size a ″ _i using the following formula (1).
a ″ _i = a _i · (γ / (sin ² θ _i + γ ² · cos ² θ _i ) ^1/2 ) ^1/2 (1)
In the formula (1), γ is an aspect ratio of nonmetallic inclusions on the reference plane. The secondary conversion step is
Calculating a major axis l _i and a minor axis s _i of a non-metallic inclusion having an aspect ratio γ projected from the primary projection dimension a ″ _i using the following formula (2) and formula (3); ,
Formula (4), (5) and 'used, the aspect ratio γ from the major axis l _i and the minor axis s _i' Aspect ratio of nonmetallic inclusions γ in the evaluation region major axis l _'i nonmetallic inclusions And calculating a minor axis s ′ _i ;
Using the following equation (6) and (7), the longer diameter l _'i and the minor axis s'' diameter l of non-metallic inclusions' aspect ratio γ projected onto the evaluation plane from _{i 'i} and the minor axis s '' calculating _i ;
The following formula (8) is used to calculate the evaluation surface projection dimension a ′ _i from the major axis l ″ _i and the minor axis s ″ _i . Cleanliness evaluation method.
a ″ _i = (l _i · s _i · π) ^1/2 (2)
γ = l _i / s _i (3)
l ′ _i = (s _i ² · l _i ) ^1/3 · (γ ′) ^2/3 (4)
s ′ _i = ((s _i ² · l _i ) / γ ′) ^1/3 (5)
l ″ _i = (s _i ² · sin ² θ ′ + (γ ′) ² · cos ² θ ′) ^1/2 (6)
s ″ _i = s ′ _i (7)
a ′ _i = (l ″ _i · s ″ _i · π) ^1/2 (8) A cleanliness evaluation method for a specific evaluation surface in a forged steel product,
Measuring the maximum non-metallic inclusion size a _i (i = 1 to n) on n inspection surfaces having the same angle to the forging direction;
Calculating an estimated maximum non-metallic inclusion dimension a _max on the inspection surface from the maximum non-metallic inclusion dimension a _i using an extreme value statistical method;
Using the angle θ with respect to the forging direction of the inspection surface and the angle θ ′ with respect to the forging direction of the evaluation surface, the estimated maximum non-metallic inclusion size a _max on the inspection surface is the estimated maximum non-metallic inclusion size on the evaluation surface. a method of converting to a ′ _max . A method for evaluating the cleanliness of a forged steel product.