JP5031221B2 - Method for preventing surface cracks in cold forging - Google Patents

Description

  The present invention relates to a method for preventing surface cracking of a semi-processed product and a molded product during continuous cold forging of an automobile or the like.

  Conventionally, a continuous cold forging method has been used for parts that are used in automobiles and require toughness.

And in order to prevent the crack which generate | occur | produces on the surface of the semi-processed goods and molded product at the time of continuous cold forging, improvement of a forging raw material, a lubricant, etc. is performed. As a method for determining the presence or absence of surface cracks before the forging die is manufactured, Patent Document 1 sets a plurality of trace points for expressing a fiber flow line inside the workpiece to be forged by the die. Calculate the displacement of the trace point accompanying the deformation of the workpiece, and connect the trace points after the displacement to represent the fiber flow line after the deformation of the workpiece. A new trace point can be set between points, and the forging process can be simulated in a short time. Analysis of the cross section perpendicular to the longitudinal direction of the material and the part that is greatly enlarged and the three-dimensional evaluation of the analysis result A forging analysis method that can also be performed is shown, and Non-Patent Document 1 shows a conditional expression for ductile fracture.
JP 2000-197942 A Moriya Oyane “Conditions for Ductile Fracture” Journal of the Japan Society of Mechanical Engineers Vol. 75, No. 639 April 1972 P596-601

  In recent years, as near-net shaping has progressed, operations at the limit points of all conditions such as forging materials, lubrication conditions, number of forging processes, etc. are required. It is impossible to cope with the error method, and it is necessary to predict the presence or absence of surface cracks due to CAE with a high degree of accuracy in advance before examining the actual system.

  However, although the forging analysis method is shown in the above Patent Document 1, the limit condition of the surface crack is not clear, and even if the “conditional expression of ductile fracture” according to the above Non-Patent Document 1 is used, Since the part where the crack occurs and the part where the stress is high may be different, it is difficult to accurately predict the presence or absence of the surface crack before manufacturing the forging die, and an alternative method is required.

  The present invention has been made to achieve the above-mentioned problems, and as a result of various studies, the presence or absence of cracks can be predicted with high accuracy in advance by using the evaluation index of the surface expansion rate (elongation rate) and the maximum principal stress. Is a method for preventing surface cracking in continuous cold forging.

1st invention for achieving the said subject is mass%, C: 0.45-0.60%, Si: 0.03-0.15%, Mn: 0.20-0.50%, B: 5-50ppm, Ti: 0.02-0.05% N ≦ 100 ppm, a method for preventing surface cracking in continuous cold forging using carbon steel after complete spheroidizing heat treatment containing other impurity elements as a forging material, the surface of a semi-processed product or a molded product in the whole forging process the surface enlargement ratio of all of the sites, and following the forging is the limit surface magnification of the material 10, when the surface enlargement ratio is 10 to 15 from more than the limit surface magnification of the forged material of It is a method for preventing surface cracking, characterized in that the maximum principal stress at a site exceeding the limit surface expansion rate is kept below the tensile strength of the forging material.

In continuous cold forging, surface cracking of a semi-processed product or a molded product can be prevented by performing process design and mold design so that the surface expansion rate is equal to or less than the limit surface expansion rate.
In addition, when the surface enlargement ratio exceeds the limit surface enlargement ratio, the surface crack of the semi-processed product or the finished product may or may not occur. Regardless of the factor, surface cracks always occur. However, in continuous cold forging using forged carbon steel after complete spheroidizing heat treatment, even if the surface enlargement ratio exceeds the limit surface enlargement ratio, if below strength sigma _B tensile maximum principal stress sites, it is possible to prevent surface cracking, a more stable forged surface if below 85% of the maximum principal stress in the site tensile strength sigma _B Obtainable.

  In order to keep the maximum principal stress value below the tensile strength, it is not limited to the process design and the die design change, but may be performed by changing the characteristics of the forging material or changing the lubricant.

According to the present invention, the surface crack in the continuous cold forging process can be accurately predicted as compared with the case where only the stress calculation value by CAE is used, and it is not necessary to repeat the trial and error. Optimum forging process design and forging die design with a minimum number can be realized, and the effects of eliminating the heat treatment process, shortening the production preparation period, and reducing the production cost of the forging die can be obtained.

  FIG. 1 is a flowchart of forging process design and forging die manufacturing according to the present embodiment.

  In FIG. 1, first, in a first operation, based on conventional experience or a database, a predetermined number of forging steps and a die shape (material shape) for each step are determined taking into account the product shape.

  Next, in the second operation, the surface expansion rate (elongation rate) ρ of all the parts of the material surface in each process is calculated by three-dimensional CAE.

Subsequently, in the third operation, the size of the surface expansion rate (elongation rate) ρ calculated in the second operation and the limit surface expansion rate (elongation rate) ρ _A in the database of the forging material are compared. And when the calculated surface enlargement ratio (elongation) [rho is a limit surface enlargement ratio less than (elongation) [rho _A, i.e. when [rho ≦ [rho _A is forging number is confirmed, it is possible undertake production of forging die . Here, the limit surface expansion ratio is the limit of the surface expansion ratio that causes cracks on the surface of the forged material steel when the surface expansion ratio exceeds a certain value in the state where the main stress on the forged material steel surface slightly exceeds the tensile strength. The value (maximum value) is a unique value that each forged steel has.

When the surface expansion rate (elongation rate) ρ is larger than the limit surface expansion rate (elongation rate) ρ _A , that is, when ρ> ρ _A , the surface expansion rate (elongation rate) ρ is the limit surface expansion rate in the fourth operation. If the ratio exceeds 1.5 times, the process returns to the first work, the shape of the portion having a large surface expansion rate (elongation rate) ρ is corrected, and the work after the second work is repeated. Surface magnification (elongation) [rho is a limit surface magnification ratio is equal to or less than 1.5 times the (elongation) [rho _A, in the fifth operation, the surface enlargement ratio (elongation) [rho is a limit surface magnification ( greater than elongation) [rho _a, calculates the maximum principal stress sigma sites of 1.5 times or less, it compares the magnitude of the tensile strength sigma _B in the database of the forging material. When the calculated maximum principal stress σ is smaller than the tensile strength σ _B (more preferably, 85% or less of the tensile strength), that is, σ <σ _B (more preferably, σ ≦ 0.85σ _B ) The number of forging steps is determined, and the production of forging dies can be started. When the maximum principal stress sigma is the tensile strength sigma is greater than _B which is calculated conversely, i.e. when the sigma> sigma _B, back to the first working surface magnification (elongation) Fix large part of the shape of ρ Repeat the second and subsequent operations.

In the third operation, when the surface enlargement ratio (elongation) [rho is having a margin against the limit surface magnification (elongation) [rho _A, ie if Ro«ro _A, in the process design in the first operation, When the surface enlargement rate (elongation rate) ρ is much larger than the limit surface enlargement rate (elongation rate) ρ _A in the third operation, that is, ρ >> ρ. _{When A} , it is necessary to increase the forging process.

  In the second operation, the surface enlargement rate (elongation rate) ρ is calculated by three-dimensional CAE. However, it can be measured by a plasticine experiment. When a prototype mold is manufactured prior to the mold and the mold shape is determined, it is effective to measure by plasticine experiment.

As described above, it is necessary to examine the surface expansion rate (elongation rate) ρ of all the parts of the material surface in each forging process, but in the actual work of the second operation in the above flow diagram, the surface expansion rate of the material surface The purpose can be achieved if CAE analysis is performed by selecting only a portion where (elongation rate) ρ is expected to be close to the limit surface enlargement rate (elongation rate) ρ _A , and the CAE analysis man-hours can be reduced.

mass%, C: 0.45-0.60%, Si: 0.03-0.15%, Mn: 0.20
˜0.50%, B: 5 to 50 ppm, Ti: 0.02 to 0.05%, N ≦: 100 ppm
In addition, carbon steel containing other impurity elements is used as a forging material with a fully spheroidized annealing material (limit surface expansion ratio ρ _A = 10, tensile strength 590 MPa), and surface expansion ratio (elongation) is used for forging process design and forging die production. Examples and comparative examples relating to continuous cold forging of a knuckle spindle to which (ratio) ρ is applied will be described below.

  FIG. 2 is a shape diagram (cross section) of a material in each step of continuous cold forging of a knuckle spindle according to an example or a comparative example of the present invention, and is based on a forging process design and a forging die manufacturing flowchart shown in FIG. The process was designed and a forging die was produced.

  In FIG. 2, reference numeral 10 denotes a knuckle spindle, which is formed by a shaft portion 20 and a ball portion 30. This knuckle spindle 10 is composed of four cold forging steps, and FIG. 1 (A) shows the first step, in which a portion of the cylindrical forging material, which is mainly a shaft portion of a product, is processed and formed.

  Fig. (B) shows the second step, where the part that will be the ball part of the product is mainly processed and molded. Fig. (C) shows the third step, and the ball part of the product is further processed and molded following the second step. Has been. FIG. 4D shows the fourth step, which is the final step, and the product shape is processed and molded.

FIG. 3 is a deformation analysis diagram by three-dimensional CAE in continuous cold forging of the knuckle spindle according to the first embodiment of the present invention. FIG. 3A is a diagram in which a part of a cylindrical material is divided into elements by a finite element method. FIG. (B) shows the state of change of the material and elements when reaching the third step shown in FIG. 2 from the center (point O) to the outer periphery (point G) of the forging material. .

  Although FIG. 3 graphically represents the deformation, it is difficult to quantitatively capture the surface enlargement rate (elongation rate) ρ by visual observation, but in the analysis by three-dimensional CAE, the surface enlargement rate (elongation) Ratio) ρ is grasped digitally, and surface cracks occur when the elongation of this element exceeds a predetermined value, and the maximum stress calculation value and the surface crack generation site as in the conventional ductile fracture condition equation Therefore, the surface crack can be evaluated very accurately. C part of FIG. 3 has shown the site | part from which a surface expansion rate becomes a peak.

  FIG. 4 is an outline diagram showing the surface shape of the punch in the third step of the continuous cold forging according to the example of the present invention and the comparative example.

  FIG. 5 is a graph showing the surface expansion rate (elongation rate) ρ in continuous cold forging according to Example 1 and Comparative Example 1, and corresponds to Example 1 and Comparative Example 1 shown in Table 1. Yes, it shows the surface expansion rate (elongation rate) ρ in the radial direction of the inner surface of the ball portion at the end of the third step of the knuckle spindle having a different shape of the corner R of the forging die. The central portion (point O) of B) and the end portion (about 80 mm) of the abscissa correspond to the right end portion (point G) in FIG.

  In FIG. 5, the surface expansion rate (elongation rate) ρ can be continuously grasped along the surface shape of the inner surface of the ball portion, and it can be clearly grasped at which part the surface crack occurs.

  Table 1 lists the relationship between the surface expansion rate (elongation rate) ρ, the maximum principal stress σ, and the surface cracks according to Examples and Comparative Examples.

In Table 1, in Example 1, as shown in FIG. 2, the corner radius of the knuckle spindle is 15 mm, and the surface enlargement ratio (elongation rate) ρ is 7.1 in the third step and 7 in the fourth step. 6 and no surface cracks occurred. Incidentally, the maximum principal stress σ is 100 MPa for the third step and 640 MPa for the fourth step.

Example 2 was formed using the same forging die used in Comparative Example 1 (the corner R was 1 mm as shown in FIG. 2), but after the second forging process was completed, the bonder The friction coefficient μ is 0.1 even though the corner R is as small as 1 mm by processing and strengthening the lubrication.
Since 0 is smaller, third, there surface enlargement ratio is 10 or less of 9.0,9.4 and limitations surface enlargement ratio [rho _A, respectively, in the fourth step, surface cracking does not. Incidentally, looking at the maximum principal stress σ, the third step is −20 MPa, the fourth step is 740 MPa, which exceeds the tensile strength σ _B of 590 MPa (elongation).

From Examples 1 and 2, it can be seen that when the surface expansion ratio ρ is equal to or less than the limit surface expansion ratio ρ _A (ρ <10), no surface cracks occur regardless of the maximum principal stress σ.

Next, Comparative Example 1 consists of the same four steps as Example 1, and uses the same material steel as Example 1, but the shape of the forging die is different (corner R = 1 mm). The surface expansion rate (elongation rate) ρ in 3 steps exceeds 11.8 and the critical surface expansion rate ρ _A of 10, but the maximum principal stress is as small as 210 MPa (σ ≦ 0.85σ _B ). There is no surface cracking. However, in the fourth step, the surface enlargement ratio ρ is 12.2 which is equivalent to the third step, but the maximum principal stress σ exceeds 860 MPa and the tensile strength σ _B of 590 MPa, and surface cracks are apparent in the fourth step. It has become.

Comparative Example 2 uses the same forging die used in Comparative Example 1 (the corner R is 1 mm as shown in FIG. 2) and the same material steel, and the friction coefficient is reduced to 0.25 by reducing the amount of lubrication. The surface expansion rate (elongation rate) ρ in the third step is 14.1, and the limit surface expansion rate ρ _A
However, the maximum principal stress σ is as small as 400 MPa (σ ≦ 0.85σ _B ), and there is no occurrence of surface cracks in the corner R portion. However, in the fourth step, the surface expansion ratio ρ is 14.4, which is equivalent to the third step, but the maximum principal stress σ exceeds 880 MPa and the tensile strength σ _B exceeds 590 MPa. Surface cracks are evident in the process.

Comparative Example 1, when the surface enlargement ratio from the third step of the Comparative Example 2 [rho is 15 or less than 10 limits the surface enlargement ratio ρ _A (ρ ≦ 15), the strong tensile maximum principal stress σ of If there is a margin with respect to σ _B (σ ≦ 0.85σ _B ), it is clear that surface cracks do not occur.

Comparative Examples 3, 4, and 5 use the same type of forging die used in Comparative Example 1 (the corner R is 1 mm as shown in FIG. 2) and the same material steel, and the friction amount is changed by changing the amount of lubrication. the are those respectively to 0.30,0.40,0.10, surface enlargement ratio in the third step [rho is significantly 17.3,19.6,16.7 and 10 limits the surface enlargement ratio [rho _a (Ρ> 15), surface cracks occurred in the third step. Incidentally, the maximum principal stress σ in the third step is above and below 680, 780, 400 MPa and 590 MPa of tensile strength σ _B , respectively.

Comparative Example 3, 4, and 5 (third step), the surface enlargement ratio [rho greatly exceeds the critical surface enlargement ratio ρ _A (ρ> 15) and the surface cracking occurs, in this case calculates the maximum principal stress σ It turns out that it is not too long.

Example 3 uses the same die as the forging die used in Example 1, and is formed by reducing the lubricant, and the surface enlargement rates in the third and fourth steps are 11.8 and 12.0, respectively. The limit surface enlargement ratio ρ _A exceeds 10, but there is no surface crack. Incidentally, looking at the maximum principal stress σ, the third step is 180 MPa and 85% or less of the tensile strength, and the fourth step is 520 MPa and exceeds 85% of the tensile strength. If the surface expansion ratio ρ exceeds 10 of the limit surface expansion ratio ρ _A and is in the range of 15 or less, the surface cracks occur when the maximum principal stress σ exceeds 85% of the tensile strength σ _B but does not reach 100%. There is no occurrence. In particular, when the tensile strength σ _B is 85% or less, a very good forged surface similar to the case where the surface expansion rate is the critical surface expansion rate ρ _A or less can be obtained.

  As described above, according to the present embodiment, the surface cracking rate in the continuous cold forging process is accurately predicted by the surface expansion rate (elongation rate) ρ as compared with the case where the stress calculation value by the three-dimensional CAE is used. Therefore, it is not necessary to repeat trial and error, the optimal forging process design and forging die design that can reduce the number of forging processes is possible, the heat treatment process can be eliminated, the production preparation period can be shortened, and the forging die The production cost was also reduced.

It is a flowchart of the forge process design and forge die manufacture which concern on this Embodiment. It is a shape figure of the raw material in each process of the continuous cold forging of the knuckle spindle which concerns on the Example or comparative example of this invention. It is a deformation | transformation analysis figure by three-dimensional CAE in the continuous cold forging of the knuckle spindle which concerns on Example 1 of this invention. It is an outline drawing which shows the surface shape of the punch in the 3rd process of the continuous cold forging which concerns on the Example and comparative example of this invention. It is a graph which shows the surface expansion rate (elongation rate) (rho) in the continuous cold forging which concerns on Example 1 and Comparative Example 1 of this invention.

Explanation of symbols

10 ... Knuckle spindle 20 ... Ball part 30 ... Shaft part

Claims (1)

At mass%, C: 0.45-0.60%, Si: 0.03-0.15%, Mn: 0.20-0.50%, B: 5-50ppm, Ti: 0.02-0.05%, N≤100ppm, complete with other impurity elements A method for preventing surface cracking in continuous cold forging using carbon steel after spheroidizing heat treatment as a forging material, wherein the forging material has a surface enlargement ratio of all parts of the surface of a semi-processed product or a molded product in the entire forging process. of a 10 or less which is the limit surface magnification, when the surface enlargement ratio is 10 to 15 from more than the limit surface magnification of forged material, the maximum principal stress sites exceeding該限field surface enlargement ratio A method for preventing surface cracking, wherein the forging material has a tensile strength of 590 MPa or less.

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