JP5737193B2 - Processing crack sensitivity evaluation method - Google Patents

Description

  The present invention relates to a method for evaluating the susceptibility to processing cracks generated on the surface of a material when a forged part such as a gear is manufactured by cold forging a cylindrical work material obtained by cutting a steel bar into a predetermined size.

  Parts such as gears used in automobiles and industrial machines are manufactured by hot forging a cylindrical workpiece cut into a predetermined size using a steel bar as a raw material and then cutting it. However, parts such as gears may be manufactured by cold forging for the purpose of saving process and energy, and this construction method tends to increase in the future. When a material is cold forged, the processing limit of the material becomes a problem, and the processing limit is almost always regulated by processing cracks generated in the workpiece. Therefore, superiority or inferiority in cracking during cold forging of a material, that is, making an appropriate assessment of workability, is a matter of selecting a material for the intended processing or determining suitability, or for a given material. This is extremely necessary for calculating a reasonable degree of processing or estimating the success or failure of processing.

For this reason, several test methods have been proposed as methods for evaluating the cold workability of materials. For example, in the cold upsetting test method, a cylindrical specimen is cut out from a steel bar material, the cylindrical specimen is compressed in the axial direction, the presence or absence of cracks is observed, the limit upsetting ratio is obtained, and the material is processed. This is a method for evaluating sex. Cracking occurs when a fine crack (0.5 to 1.0 mm in length) is observed for the first time, and the height of the specimen is measured to determine the cracking height (hc). Then, when the height of the first specimen is h ₀ , the limit upsetting ratio ε _hc (%) is obtained by the equation ε _hc = (h ₀ −h _c ) / h ₀ × 100 ( For example, refer nonpatent literature 1).

  In addition, steel bars have surface properties such as roll rolls and wrinkles at the time of rolling, and the rolled material, or the material obtained by annealing or bonding the rolled material, the cylinder cut without peeling these materials The same applies to the surface properties of the workpiece. Such surface properties of the material to be processed greatly affect the processing cracks. However, in conventional upsetting test methods, a small-diameter cylindrical specimen is often cut and manufactured from a steel bar material, and the cylindrical specimen does not have the surface properties of a steel bar. It is not possible to evaluate the surface processing cracks of the cylindrical workpiece that has been cut. Although it is possible to produce a cylindrical specimen cut from a steel bar material, for example, when it is as large as about 55 mmφ, depending on the strength of the steel bar, a load exceeding 15000 kN is required. There is a problem of needing.

  In addition, in a cylindrical tool test method proposed as another test method, a cylindrical test body is compressed in the axial direction by a cylindrical surface of a cylindrical tool having a V-shaped groove formed in a lattice shape on the surface (for example, non- Patent Document 2). In this method, the cold upsetting property of a material is evaluated by the ratio of the height and diameter of the specimen and the rate of decrease in the height of the minimum height when a crack occurs in the compressed specimen.

  This test method is a test method that is possible with a low load, but still requires a load that is about 1/3 of the cold upset test method. For example, a 55 mmφ test body requires a testing machine having a load capacity of over 5000 kN.

“Plasticity and processing” vol. 22, no. 241 ((1981-2), 139-144) “Plasticity and processing” vol. 18, no. 202 ((1977-11), 923-929)

Therefore, in view of such problems, the present invention has the effect of compressive load on the surface processing cracking sensitivity during cold forging of a cylindrical steel bar material produced by cutting a rolled steel bar material into a predetermined dimension. It is an object of the present invention to provide a method for evaluating the sensitivity to processing cracks on the surface of a steel bar that can be appropriately evaluated without using a large pressing device.

The inventors of the present invention have intensively studied to solve the above problems, and as a result, cut the rolled steel bar material into a predetermined size, cut through the center portion to produce a hollow specimen, By calculating the critical flatness ratio from the occurrence of cracks in the outer surface layer when flattened in the diameter direction, the cylindrical steel bar material cut into a predetermined size is compressed in the central axis direction (cold). The present invention has been completed by finding a method for evaluating the sensitivity of work cracks on the surface of steel bars that can appropriately evaluate the surface work cracks that occur during forging.

  The gist of the present invention is as follows.

(1) Cut the rolled steel bar material, produce a hollow specimen that has been cut through the center, and flatten the flat specimen when the hollow specimen is flattened in the vertical diameter direction. From the relationship with the occurrence of cracks in the outer surface layer at the horizontal position of the flat body, the critical flatness specified in the following formula is obtained, and the cylindrical steel bar material produced by cutting the rolled steel bar material A method for evaluating the cracking susceptibility of a steel bar surface, characterized in that the surface cracking property during cold forging is evaluated based on the limit flatness ratio.
Limit Enough Tairaritsu _{ε hi = (h 0 -hi)} / h 0 × 100 (%)
Here, h ₀ means the height (outer diameter) (mm) of the first hollow specimen, and hi means the height (mm) when a crack occurs in the outer surface layer in the horizontal position.

(2) The method of evaluating the cracking susceptibility of the steel bar surface according to (1) above, wherein the relationship between the outer diameter D and the inner diameter d of the hollow test body is d / D> 0.5.

(3) the hollow specimen is outer diameter from D, rolled left bars of length L, a outer diameter D, wherein the processing into a hollow specimen of internal diameter d, length L, a (1) Alternatively, the method for evaluating the sensitivity to processing cracks on the steel bar surface according to (2).

(4) The hollow test body according to any one of (1) to (3) above, wherein the relationship between the outer diameter D and the length L is such that L / D is 0.5 to 2. Method for evaluating the susceptibility to machining cracks on the surface of steel bars .

  According to the present invention, the required load of the press required for the test can be carried out with a very low load compared to the cold upsetting test, and further, it was produced by cutting a large-diameter steel bar as rolled. Including the surface properties of the cylindrical workpiece, it is possible to achieve a remarkable effect that the surface processing crack sensitivity during cold forging can be evaluated.

It is a figure for demonstrating the conventional cold upsetting test, (a) is before compressing a test body to an axial direction, (b) is a figure for demonstrating the state which the crack generate | occur | produced in the compressed test body. It is. It is a figure which shows the relationship between the compression rate (%) of a steel material (S45C), and a forge load (kN). It is a figure which shows producing the thin diameter test body for the conventional cold upsetting test. (A)-(d) is a figure which shows the state which compresses the hollow test body by the test method of this invention to a perpendicular direction (diameter direction) with a press. It is a figure which shows the relationship between a flat rate and a cumulative crack rate.

  Embodiments of the present invention will be described below.

  First, we will describe the cold upsetting test recommended by the Japan Society of Plasticity and Forging, which is a conventional method for evaluating the working limit of steel for cold forging, which is generally used in the forging industry. As shown in FIG. 1 (a), the cold upsetting test is performed by compressing the test body 1 with the end face restraining jig 2 and applying it to the side surface of the compressed test body 3 as shown in FIG. 1 (b). In this method, the compression ratio at which the crack 4 occurs is used as a scale.

  However, recent steel materials have a critical compressibility of about 75%, and as shown in the relationship between the compressibility (%) of steel material (S45C) and forging load (kN) in FIG. In order to compress, a testing machine with high load capacity is required. For example, a test machine having a load capacity exceeding 12000 kN is required for a large-diameter 55 mmφ cylindrical specimen. For this reason, it is difficult to perform a test using a cylindrical specimen obtained by cutting a large-diameter steel bar. Therefore, normally, as shown in FIG. 3, a test specimen 1 having a small diameter of about 14 mmφ is cut out from the large-diameter 55 mmφ bar steel 5 and used for a cold upsetting test.

In the cold upsetting test, in view of the necessity of a testing machine with a high load capacity and the inability to evaluate the cracking susceptibility of the steel bar surface considering the surface properties of the workpiece material, the present invention provides a load capacity. We have intensively researched a method that can evaluate the processing limit of large-diameter materials that are rolled at low load without using a high-strength testing machine, and can evaluate the cracking susceptibility of the steel bar surface considering the surface properties of the workpiece .

  First, in order to enable tests that can evaluate the surface processing cracking susceptibility of large-diameter materials at low loads, we conceived of using a hollow specimen and tried to compress the hollow specimen in the axial direction. In the test, the hollow specimen was buckled (buckled) in the middle, and it was not possible to evaluate the processing crack of the material.

  Therefore, further research was conducted, and a test was carried out to compress the hollow specimen in the diameter direction. As a result, we conducted a flattening test to compress the hollow specimen in the vertical diameter direction, and found that it was possible to effectively evaluate the surface processing cracking susceptibility of a cylindrical workpiece made by cutting a large diameter material. Completed the invention.

The present invention cuts a large-diameter steel bar material that has been rolled, produces a hollow test body that has been cut through the center, and flattened the flat test body when the hollow test body is flattened in the vertical diameter direction. and flat rate, the relationship between the crack occurrence at the outer surface in the horizontal position of the flat body, seeking limit flat rate, by cutting the steel bar material-rolled based on該限boundary flat rate It is characterized by evaluating the surface work cracking property during cold forging of the fabricated cylindrical steel bar material.

  Hereinafter, a test method for evaluating the surface processing cracking susceptibility of a cylindrical workpiece made by cutting a rolled steel bar material with a large diameter as rolled according to the present invention will be described.

  The method of the present invention can be used regardless of the composition of the material and the presence or absence of heat treatment. JIS S53C steel (0.53% C, 0.24% Si, 0.77% Mn, 0.018% P, 0.005% S), a steel generally used for cold forging A 45 mmφ bar steel manufactured by rolling and spheroidizing annealing (740 ° C. × 7 hr—slow cooling) at a predetermined area reduction rate using a bar-shaped material was used as a test specimen.

  In the present invention, a hollow test body having a length of 50 mm, an inner diameter of 36 mm, and an outer diameter of 45 mm (inner diameter / outer diameter = 0.80) was prepared from the test material, and a flat test was performed.

In the flat test, a flat test was performed in which the hollow test body was compressed into a flat body as shown in FIGS. Here, the initial height h ₀ (corresponding to the outer diameter) and the height h after compression (h ₀ −h) / h ₀ × 100 (%) were defined as the flatness ratio (%). . Various changes were made in the flatness ratio, and the occurrence of cracks in the outer surface layer (side surface) at the horizontal position of the flat body at that time was observed visually or with a 10-fold magnifier. The occurrence of cracks was defined as the flatness at the time of crack occurrence when a fine crack having a length of 0.5 to 1.0 mm was observed for the first time.

A flat test was conducted on a plurality of hollow specimens to investigate the occurrence of cracks, and the relationship between the flat rate and the cumulative crack rate was determined. The results are shown in FIG. As shown in FIG. 5, in this test, the cumulative cracking rate was 50% when the flattening rate was 66%, and the flattening rate at this time was evaluated as the limiting flattening rate. The required load required when the flatness ratio was 66% was about 200 kN.

In order to determine the limit flatness, at least 6 and preferably 6-30 hollows with a flatness greater than the flatness observed for the first time that a hollow specimen was cracked in a flat test. A flat test is performed on the specimen to determine the number of cracks. In the example shown in FIG. 5, a flat test was performed on a hollow specimen, and one crack was observed at a flat rate of 64%, and two cracks were observed at a flat rate of 66%. In this example, two cracks were observed at a rate of 68%, and one crack was observed at a flat rate of 70%. Then, the flatness ratio 66%, which is 2 + 1 among the number of cracks, was evaluated as a cumulative cracking ratio 50%, and the critical flatness ratio was 66%.

Therefore, the critical flattening ratio ε _hi (%) is the critical flatness when the height (outer diameter) of the first hollow specimen is h ₀ and the height when the cumulative cracking rate is 50% is hi. The average ratio ε _hi = (h ₀ −hi) / h ₀ × 100 (%).

  On the other hand, as a comparative example, a cylindrical test body having an outer diameter of 45 mmφ and a length of 67.5 mm obtained by cutting a rolled steel bar was prepared, and a conventional cold upsetting test was performed. As a result, the limit upsetting rate was 72%, and the required load was 6970 kN. Further, a cylindrical test body having an outer diameter of 25 mmφ and a length of 37.5 mm was cut out from the steel bar, and a conventional cold upsetting test was performed. As a result, the limit upsetting rate was 74%, and the required load was 2200 kN.

  As shown in the examples to be described later, when comparing the flattening test results according to the present invention and the conventional cold upsetting test results, in the limiting flattening ratio in the present invention and the conventional limiting upsetting rate, The superiority and inferiority of the work cracking property of the workpiece material showed almost the same tendency, and it was confirmed that the work cracking property of the workpiece material could be evaluated by the limit flatness. In the flat test of the present invention, the required load of the hydraulic press required for the test can be carried out with a very low load compared to the cold uptake test, and the surface of the steel bar that affects the work cracking It is possible to evaluate the processing cracking susceptibility of work materials including properties.

  Also, in the flat test, cracks can be detected in the outer surface layer in the horizontal position of the flat body after compressing the hollow test body in the vertical diameter direction because the thickness of the hollow test body has a large effect. It is important that the thickness of the specimen (the relationship between the outer diameter D and the inner diameter d) be within a predetermined range.

  In other words, if the wall thickness is too thick, the hollow part is brought into close contact early and flattening is only possible to a flatness ratio of less than 60%, and the vertical part is more severe with respect to cracking than the horizontal part. Cannot be implemented. In order to make it possible to test a material with excellent workability, the relationship between the outer diameter D and the inner diameter d of the hollow specimen is preferably d / D> 0.5. Further, since the range of application becomes wider when d / D is higher, it is more preferable to set d / D = 0.75 to 0.85.

  In addition, the hollow test body is obtained by cutting a rolled steel bar (outer diameter D) into a solid body of the outer diameter D and length L. From this solid body, the outer diameter D, inner diameter d, length L It is manufactured by processing into a hollow test body. Here, the length L is not particularly limited, but is preferably L / D = 0.5-2. When L / D exceeds 2, it is not preferable because it is necessary to make the press apparatus large. Further, when L / D is less than 0.5, it is difficult to stably hold the specimen during the breathing.

  Hereinafter, based on an Example, this invention is demonstrated concretely.

  In the test of this example, in order to evaluate the work cracking susceptibility of the outer surface layer of the steel bar material, a flat test of the hollow cylindrical specimen and an upsetting test of the steel bar material were performed.

(Material)
A 45 mmφ rolled material of JIS S45C (0.46% C, 0.26% Si, 0.76% Mn, 0.020% P, 0.010% S) was used as the material. In order to investigate the influence of the outer surface of the material, a rolled material was produced in which the surface properties of the outer surface layer were changed. As the rolled material, a rolled material was prepared when the finish rolling roll was changed to a) a new article, b) immediately before replacement, and c) an intermediate part thereof. Then, the following experiment was performed using a sample obtained by subjecting this rolled material to a spheroidizing annealing treatment (740 ° C. × 7 hr—slow cooling) as a softening treatment.

(Test specimen)
In the examples of the present invention shown in Tables 1 and 2, the outer surface layer was received and not machined, and machined in the inner diameter side and in the length direction to be processed into hollow test bodies of various shapes.

  The 45 mmφ test specimens of Comparative Examples 1 to 3 in Table 2 were cut into a cylindrical shape by cutting a 45 mmφ round bar material. The 14 mmφ test specimens of Comparative Examples 4 to 6 are prepared from the r / 2 part of the 45 mmφ round bar material (so that the position inside 1/4 of the diameter from the surface of the round bar material is the center of the test specimen). Collected by machining. The roughness of any machined surface was such that the 10-point average roughness Ra according to JIS B0601 ('82) was 2 to 3 µm.

(Forging equipment)
A hydraulic press with a maximum load capacity of 10,000 kN was used for the test. Under any of the test conditions, the reduction speed of the jig was constant at 50 mm / s.

  As a jig, a flat jig was used in the flat test, and a constraining jig with concentric grooves was used in the upsetting test. And the maximum load (required load) for every reduction was measured with the load cell.

(Simple flat test)
The flattening test method of the present invention example was carried out as follows.

  First, as a preliminary test, an average flattening ratio of cracks was obtained. Here, the flatness was reduced to 40% at the first time, and the flatness was added by 2% to the same specimen until further cracking occurred.

  Whether or not cracking occurred was observed with a magnifying glass having a magnification of 10 times. The horizontal portion of the test specimen after flattening was determined as a crack when the crack length was 0.5 to 1.0 mm.

  As shown in Table 1, in the examples in which the presence / absence of crack occurrence at each flatness ratio was tested, in “Preliminary Example 1-0” of the preliminary test, cracking occurred at a flatness ratio of 68%. The required load was 220 kN. And this test tested n number as 6. The initial flatness was 52%, which was about 15% lower than the flatness determined in the preliminary test. In the examples, as in “Examples 1-1 to 1-6”, the flatness was increased by 2% from 52%.

  The flatness ratio at which cracking occurred was 1 at 64%, 2 at 66%, 3 at 68%, and n / 2 (cumulative cracking ratio 50%) at an average flatness of 66%. The piece broke. Therefore, the critical flat rate is 66%.

  Further, the limit flatness of each specimen was obtained by the procedure described above. The results are shown in Table 2.

  Inventive Examples 1 to 3 are the results when the outer surface roughness was changed at the inner diameter d / outer diameter D = 0.76 of the test specimen, and the result that the critical flatness decreased as the surface roughness became rougher. Obtained. This was the same as the order of crack sensitivity when the parts were actually forged.

  Examples 4-6 and 7-9 of the present invention are the results when the shape of the specimen (inner diameter d) is changed. Similar to Examples 1-3 of the present invention, the limit flatness decreases as the surface roughness increases. was gotten.

  The required load in the example of the present invention is 200 kN or less, and the crack sensitivity evaluation with a small forging device is possible.

  Comparative Examples 1 to 3 were upsetting tests in a state where the outer surface layer of the round bar material was left, and the result that the critical flatness decreased as the surface roughness became rough was obtained. This was the same as the order of crack sensitivity when the parts were actually forged. However, the required load reaches close to 7000 kN, and crack sensitivity cannot be evaluated without a large forging device.

  Comparative Examples 4 to 6 are upsetting tests when machined to φ14. Although the required load is small, the outer surface layer of the round bar material has been removed, so the limit upsetting ratio is almost the same crack sensitivity as 75 to 76%, which is inconsistent with the crack sensitivity when the parts are actually forged. It is not suitable as an evaluation method.

  Comparative Examples 7 to 9 are flattening tests, but are outside the range defined by the present invention, i.e., inner diameter d / outer diameter D = 0.44. In this case, since the inner diameter portion is crushed early, no crack is generated, and the limit flatness ratio cannot be obtained.

1 Specimen 2 End face restraint jig 3 Compressed specimen 4 crack

Claims (4)

The rolled steel bar material is cut, a hollow test body is cut through the center, and the flatness of the flat body is flattened when the hollow test body is flattened in the vertical diameter direction. From the relationship with the occurrence of cracks in the outer surface layer in the horizontal position of the flat body, the critical flatness specified in the following formula is obtained, and the cold work of the cylindrical steel bar material produced by cutting the rolled steel bar material. the surface treatment cracking during forging, machining crack sensitivity evaluation method steel bar surface and evaluating, based on the limits strange Tairaritsu.
Limit Enough Tairaritsu _{ε hi = (h 0 -hi)} / h 0 × 100 (%)
Here, h ₀ means the height (outer diameter) (mm) of the first hollow specimen, and hi means the height (mm) when a crack occurs in the outer surface layer in the horizontal position. 2. The method for evaluating the cracking susceptibility of a steel bar surface according to claim 1, wherein the relationship between the outer diameter D and the inner diameter d of the hollow specimen is d / D> 0.5. The hollow test body outer diameter from D, rolled left bars of length L, a outer diameter D, an inside diameter d, characterized by machining a hollow specimen of length L, a claimed in claim 1 or 2 Of evaluating cracking susceptibility of steel bar surface . The said hollow test body is the cracking sensitivity of the steel bar surface in any one of Claims 1-3 characterized by the relationship between outer diameter D and length L, L / D being 0.5-2. Evaluation method.