JP2023120677A - Inclusion evaluation method - Google Patents

Abstract

Kind Code: A1 A method for evaluating inclusions is provided that enables stable evaluation of a test piece made of a metal material having a hardness of less than 400 HV.
A test piece made of a metallic material containing non-metallic inclusions of 10 μm or more is heat-treated to have a hardness of less than HV400, hydrogen is penetrated into the test piece having a hardness of less than HV400, and the hydrogen is introduced into the test piece. A destructive test is performed on the test piece to cause a fracture originating from the nonmetallic inclusion of 10 μm or more, and the size of the nonmetallic inclusion that is the origin of the fracture is measured.
[Selection drawing] Fig. 5

Description

The present invention relates to an inclusion evaluation method for evaluating inclusions contained in a metal material.

In metallic materials, it is known that non-metallic inclusions contained therein serve as starting points for fatigue fracture. Therefore, it is important to evaluate nonmetallic inclusions contained in metallic materials.

As a conventional inclusion evaluation method, there is a method in which a tensile test as a destructive test is performed on a test piece made of a metal material into which hydrogen is introduced, as in Patent Document 1. In this inclusion evaluation method, a non-metallic inclusion that is the starting point of fracture is identified by a tensile test, and the dimensions are measured and evaluated.

In such a conventional inclusion evaluation method, the intrusion of hydrogen makes it easier to cause fracture starting from nonmetallic inclusions in a tensile test, and the evaluation of nonmetallic inclusions can be performed quickly. can also be ensured.

However, it is a test piece made of a metallic material with a hardness of HV 400 or more that is more likely to cause breakage originating from non-metallic inclusions due to penetration of hydrogen. For this reason, conventional inclusion evaluation methods require a large-sized tensioning machine for destructive tests such as tensile tests. Therefore, there has been a demand for a method of easily performing a destructive test with a smaller device.

JP 2009-65789 A

The problem to be solved is that a large testing machine was required for the destructive test.

In the present invention, a test piece made of a metallic material containing nonmetallic inclusions of 10 μm or more is heat-treated to have a hardness of less than HV400, hydrogen is penetrated into the test piece having a hardness of less than HV400, and the hydrogen is penetrated. A method for evaluating inclusions, wherein a destructive test is performed on the test piece, and the size of the nonmetallic inclusion that is the starting point of the fracture is measured. I will provide a.

Since the present invention performs a destructive test on a test piece made of a metal material having a hardness of less than HV400, the destructive test can be easily performed using a smaller testing machine.

FIG. 1 is a side view schematically showing a test piece used in an inclusion evaluation method according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship between the tempering temperature and the hardness of the test piece according to the example. FIG. 3 is a conceptual diagram showing hydrogen charging to a test piece according to an example. FIG. 4 is a conceptual diagram showing a tensile test on the test piece according to the example. FIG. 5 is a conceptual extreme value statistical graph of dimensions of non-metallic inclusions according to an example. FIG. 6 is a graph showing the distribution of equivalent circle diameters for test pieces with hardnesses of HV600, HV500, and HV370.

The purpose of the present invention is to easily perform a destructive test with a smaller testing machine. For a test piece made of a metal material containing non-metallic inclusions of 10 μm or more, the hardness is made less than HV400 by heat treatment, and hydrogen is penetrated. It was realized by conducting a destructive test.

That is, in the inclusion evaluation method, a test piece 3 made of a metal material containing nonmetallic inclusions 1 of 10 μm or more is heat-treated to have a hardness of less than HV400, hydrogen is introduced into the test piece 3 having a hardness of less than HV400, and hydrogen is removed. A destructive test is performed on the intruded test piece 3, the test piece 3 is caused to break starting from a non-metallic inclusion 1 of 10 μm or more, and the size of the non-metallic inclusion 1 that is the starting point of the breaking is measured. do.

In the inclusion evaluation method, the type of non-metallic inclusion that caused the fracture to start may be identified.

Further, in the inclusion evaluation method, the distribution function of the dimensions of the non-metallic inclusion 1 that is the starting point of the fracture may be obtained, and the cleanliness of the metal material may be evaluated by this distribution function.

Moreover, the test piece 3 may have an unprocessed outer peripheral surface 3a cut out from a wire rod and configured by the outer peripheral surface of the wire rod. In this case, the position from the outer peripheral surface 3a of the test piece 3 of the nonmetallic inclusion 1, which is the starting point of the fracture, is measured.

FIG. 1 is a side view schematically showing a test piece used in an inclusion evaluation method according to an embodiment of the present invention.

In the inclusion evaluation method of the present embodiment, a test piece 3 made of a metallic material containing nonmetallic inclusions 1 (FIG. 4) of 10 μm or more is heat-treated to have a hardness of less than HV400.

The test piece 3 of this example is cut out from spring steel as a metal material, for example, a wire rod of SAE9254. This test piece 3 has a rod shape with a circular cross section according to the shape of the wire, and the outer peripheral surface 3a is an unprocessed surface that is the same as the outer peripheral surface of the wire. Both sides of the test piece 3 in the axial direction constitute gripping portions 4 . The axial direction means the direction along the axis of the test piece 3 .

In this embodiment, the axial length of the test piece 3 is 150 mm, the diameter of the test piece 3 is 9.8 mm, and the distance between gauge points and the axial length of the grip portion 4 are each 50 mm.

However, the shape and size of the test piece 3 are not limited to these. For example, a JIS No. 4 test piece or the like may be used. As the metal material, any metal material other than spring steel may be used as long as it contains non-metallic inclusions 1 of 10 μm or more.

Whether or not the test piece 3 contains nonmetallic inclusions 1 of 10 μm or more is unknown before the inclusion evaluation method is applied. If so, the nonmetallic inclusions 1 having a size of 10 μm or more will be included.

Any heat treatment may be used as long as it results in the hardness of the test piece 3 being less than HV400, and an appropriate heat treatment such as tempering, annealing, normalizing, or quenching is employed depending on the metal material. The heat treatment in this example is tempering, and the hardness of the test piece 3 having a hardness of HV400 or more is reduced to less than HV400.

FIG. 2 is a graph showing the relationship between tempering temperature and test piece hardness.

As shown in FIG. 2, when the test piece 3 is tempered at 400 degrees, 455 degrees, 580 degrees and 700 degrees, the hardness of the test piece 3 becomes HV600, HV500, HV370 and HV280, respectively. In this example, the hardness of the test piece 3 is set to HV370 by tempering at 580 degrees. The tempering time is approximately 30 minutes. The tempering time is an example, and can be appropriately set according to the material, tempering temperature, hardness, etc. of the test piece 3 . In FIG. 2, straight lines are approximate straight lines.

The hardness of the test piece 3 may be less than HV400, so it may be set to HV280, or may be tempered at about 570 degrees based on the approximate straight line to bring it closer to HV400.

After making the hardness of the test piece 3 less than HV400 by heat treatment, the test piece 3 is impregnated with hydrogen. In the following, the entry of hydrogen is referred to as "hydrogen charging".

FIG. 3 is a conceptual diagram showing the charging of the test piece 3 with hydrogen.

Hydrogen charging is performed, for example, by immersing the test piece 3 in a hydrogen charging solution 5 for a predetermined period of time, as shown in FIG. For example, the test piece 3 is immersed in a 20 mass % ammonium thiocyanate aqueous solution at 50° C. for 48 hours.

The hydrogen charging method is not limited to this, and for example, a method of exposing the test piece 3 to hydrogen gas, or immersion in an electrolytic solution such as an aqueous solution of sodium chloride and ammonium thiocyanate or an aqueous solution of sulfuric acid and arsenous acid. There is a method of applying current while

Alternatively, the test piece 3 may be formed after the metal material is charged with hydrogen. In this case, the metal material before hydrogen charging is tempered to less than HV400.

When such hydrogen charging and tempering are performed on the test piece 3 containing the nonmetallic inclusions 1 of 10 μm or more, the largest nonmetallic inclusions 1 in the test piece 3 of 10 μm or more are removed in the next tensile test. Destruction from the starting point is more likely to occur.

A tensile test is performed on the hydrogen-charged test piece 3 to cause fracture in the test piece 3 starting from nonmetallic inclusions 1 of 10 μm or more. The tensile test is preferably performed after charging with hydrogen, but may be performed during charging with hydrogen. Moreover, instead of the tensile test, other destructive tests such as a fatigue test and an impact test may be performed.

FIG. 4 is a conceptual diagram showing a tensile test on the test piece 3. FIG.

In the present embodiment, in the tensile test, both sides of the test piece 3 are held and pulled at a tensile speed of 20 mm / min, and a fracture originating from a nonmetallic inclusion 1 of 10 μm or more between the gauge points of the test piece 3 is caused. . In addition, the fracture originating from the nonmetallic inclusion 1 refers to the fracture in which the nonmetallic inclusion 1 serving as the origin of the fracture is exposed on the fracture surface 7 of the test piece 3 .

In this tensile test, in this example, the hardness of the test piece 3 before hydrogen charging is less than HV400, so compared with the case where the hardness before hydrogen charging is HV400 or more, the tester (not shown) load is small. As a result, when the hardness before hydrogen charging is HV400 or more, the test can be easily performed with a smaller tester, or the tester can be protected.

After the fracture of the test piece 3, the type of the nonmetallic inclusion 1 that is the starting point of the fracture is identified. The type of non-metallic inclusions 1 in this example is Al--Ca--Si--Mg--O system. However, the type of nonmetallic inclusions 1 differs depending on the metal material.

Identification here means specifying the type of the non-metallic inclusion 1 with certain certainty. Therefore, in addition to the identification by directly detecting the component of the non-metallic inclusion 1, it is also possible to identify it indirectly.

In indirect identification, for example, a test piece 3 made of the same kind of metal material is subjected to a fatigue test without hydrogen charging, and the type of non-metallic inclusion that is the starting point of the fracture is specified. It may be assumed that the nonmetallic inclusions 1 according to the inclusion evaluation method of this embodiment are of the same type as the nonmetallic inclusions in the fatigue test.

Further, when applying the inclusion evaluation method to a plurality of test pieces 3 of the same type, the components of the non-metallic inclusions 1 are detected for some of the test pieces 3, and the fracture starting point is detected for the remaining test pieces 3. It may be presumed that the nonmetallic inclusions 1 formed are of the same kind as the nonmetallic inclusions 1 of some of the test pieces 3 whose components have been detected.

Furthermore, when applying the inclusion evaluation method to a plurality of test pieces 3 of the same kind, a distribution straight line 9 described later is obtained, the components of the nonmetallic inclusions 1 are detected for some test pieces 3, and the nonmetallic inclusions When the inclusion 1 is located in the confidence interval of the distribution straight line 9, it is the same kind as the non-metallic inclusion 1 of the part of the test pieces 3 that detected the component of the non-metallic inclusion of the remaining test pieces 3. can be estimated.

Moreover, as long as the dimension of the nonmetallic inclusions 1 is 10 μm or more, it may be assumed that the nonmetallic inclusions 1 are of the same type. In other words, in this embodiment, it is possible not to identify the nonmetallic inclusions 1 substantially.

Before or after such identification, or alternatively after fracture of the test piece 3, the dimensions of the non-metallic inclusions 1 are measured. In this example, the dimensions of the non-metallic inclusions 1 are measured by observing the fracture surface using an electron microscope (SEM) and measuring the major axis, minor axis, and circle equivalent diameter.

The equivalent circle diameter is the diameter of a circle having the same area as the nonmetallic inclusion 1 . Instead of the equivalent circle diameter, the average diameter of the long and short diameters may be measured as the size of the nonmetallic inclusion 1 .

Thus, in this embodiment, it is possible to reliably measure the size of the largest nonmetallic inclusion 1 in the test piece 3 that was the starting point of the fracture, and the test piece 3 made of a metallic material having a hardness of less than HV400 Stable evaluation can be performed.

Further, in this embodiment, the position of the largest non-metallic inclusion 1 in the test piece 3, which is the starting point of fracture, from the outer peripheral surface 3a of the test piece 3 is measured. This position is obtained as a distance in the radial direction. Here, the outer peripheral surface 3a of the test piece 3 consists of the outer peripheral surface of a wire. Therefore, in this embodiment, it is possible to obtain the positional information of the non-metallic inclusions 1 from the outer peripheral surface of the wire. This positional information cannot be obtained by the conventional method of processing a material into test piece shapes of various standards.

In the evaluation of this embodiment, the distribution function of the dimensions of the non-metallic inclusions 1 thus measured is obtained, and the cleanliness of the metal material is evaluated using this distribution function. Specifically, a distribution straight line as a distribution function is obtained using extreme value statistics.

In obtaining the distribution function, a plurality of test pieces 3 are subjected to tempering, hydrogen charging, and tensile testing, and the dimensions of the non-metallic inclusions 1 that are the origin of fracture are measured. Then, as shown in FIG. 5, the vertical axis is the cumulative probability, and the horizontal axis is the equivalent circle diameter of the maximum inclusion, and an extreme value statistical graph plotting the dimensions of the non-metallic inclusion 1 that was the starting point of the fracture is generated. . Note that FIG. 5 only conceptually shows the extreme value statistics graph.

Based on this extreme value statistic graph, a distribution straight line 9 can be obtained as a regression straight line. By using this distribution straight line 9, the size of the largest non-metallic inclusion 1 in the metallic material can be predicted. That is, the cleanliness of the metal material can be evaluated. Cleanliness refers to the degree of non-metallic inclusions 1 contained in the metallic material. In this embodiment, cleanliness is determined by the size of the largest non-metallic inclusion 1 in the metallic material.

In this manner, the inclusion evaluation method of the present embodiment can accurately predict the maximum nonmetallic inclusion 1 as in the fatigue test. That is, the inclusion evaluation method can break the test piece 3 made of a metal material having a hardness of less than 400 HV, starting from the largest non-metallic inclusion 1 in the test piece 3 . By measuring the dimension of the non-metallic inclusion 1 that is the starting point of the fracture, stable evaluation can be performed.

FIG. 6 is a graph showing the distribution of equivalent circle diameters for test pieces 3 with hardnesses of HV600, HV500, and HV370.

This FIG. 6 graphs the equivalent circle diameters of the nonmetallic inclusions 1 measured by tempering, hydrogen charging, and tensile testing a plurality of test pieces 3 with hardnesses of HV600, HV500, and HV370. It is. The number of samples for HV600, HV500, and HV370 is 45, 15, and 10, respectively.

The vertical axis in FIG. 6 is the equivalent circle diameter, the horizontal axis is the hardness of the test piece 3, and the numerical values in the graph indicate the average equivalent circle diameter. Also, the error range in the graph indicates the range between the maximum and minimum values.

As shown in FIG. 6, the maximum value, minimum value, and average value of the dimensions of the nonmetallic inclusions 1 are approximately the same for all of HV600, HV500, and HV370. The measurement of non-metallic inclusions 1 of HV400 or more is stable.

In HV370, the maximum dimension of the nonmetallic inclusions 1 is 43 μm and the minimum dimension is 13 μm. Like this HV370, when the dimension of the nonmetallic inclusion 1 is 13 μm to 43 μm, even if the hardness of the test piece 3 is less than HV400, the fracture originating from the maximum nonmetallic inclusion 1 stably and make it possible to measure the dimension of the non-metallic inclusion 1 that is the starting point.

This tendency can be seen in the range where the size of the nonmetallic inclusion 1 is 10 μm or more. The upper limit of the size of the non-metallic inclusions 1 is not limited as long as the non-metallic inclusions 1 can be contained in the test piece 3. Although not shown, for example, even if the size of the non-metallic inclusions 1 is 500 μm, the same trend can be seen.

1 non-metallic inclusion 3 test piece

Claims (4)

A test piece made of a metal material containing non-metallic inclusions of 10 μm or more is heat treated to have a hardness of less than HV400,
Hydrogen is penetrated into the test piece having a hardness of less than HV400,
A destructive test is performed on the test piece in which the hydrogen is introduced, and the test piece is caused to break starting from the nonmetallic inclusions of 10 μm or more,
measuring the dimensions of the non-metallic inclusion that was the starting point of the fracture;
Inclusion evaluation method. The inclusion evaluation method according to claim 1,
identifying the type of non-metallic inclusion that was the starting point of the fracture;
Inclusion evaluation method. The inclusion evaluation method according to claim 1 or 2,
Obtaining a distribution function of the size of the non-metallic inclusion that was the starting point of the fracture, and evaluating the cleanliness of the metal material using this distribution function.
Inclusion evaluation method. The inclusion evaluation method according to any one of claims 1 to 3,
The test piece is cut from a wire and has an unprocessed outer peripheral surface constituted by the outer peripheral surface of the wire,
Measuring the position from the outer peripheral surface of the test piece of the non-metallic inclusion that became the starting point of the fracture;
Inclusion evaluation method.

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