JPH0695064B2 - Local overhang test method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a test method for evaluating, in particular, "local overhangability" among the forming characteristics of a steel sheet.

<Prior art and its problems> Presently, yield point, tensile strength, elongation, n value and r value are applied as typical ones as the evaluation values of formability of cold rolled steel sheet, and these values are usually used in the tensile test. Is sought by. In addition, in order to understand the molding characteristics, the limit drawing ratio, CCV
The values, Erichsen value, bulge forming height, T value, etc. are investigated, and each of these values is obtained by a Swift cup test, a conical cup test, an Erichsen test, or a hydraulic bulge test.

On the other hand, as the forming characteristics required for cold-rolled steel sheets, in addition to the usual deep drawability and overhangability that have been the standard until now,
In recent years, the "local overhanging property" required for sharpening of molded products and integration of parts with complicated shapes is drawing attention. In particular, for molded products such as automobile exterior panels, where design is important, the need for steel sheets with excellent local overhanging properties has been strongly recognized.

By the way, as an overhanging property test method for thin metal plates, JIS defines Erichsen test. In this Erichsen test, "Method A" and "Method B" are defined. In either method, however, the material flow-in occurs during the test, and therefore, these correspond to pure overhang molding. I couldn't say. Although the bulge test (Swedish bulge test) that completely prevents the material from flowing in is also known, the punch used in this test has a large contact area because it is a spherical surface. The above-mentioned "local overhanging property" could not be evaluated.

In addition, the hydraulic bulge test is known as a method for evaluating the pure overhanging property.However, in this test, the overhanging of the material occurs on the entire surface in contact with the liquid. It was not possible to evaluate the overhanging property, but rather it was often used to evaluate the ductility of materials under biaxial tension.

Therefore, conventionally, the "local overhanging property" of a material is evaluated by estimating it based on the elongation and n value obtained in a tensile test, or by actually using a model die or the like to actually form it on an actual machine. I could only do it.

Therefore, an object of the present invention is to provide a test method capable of easily evaluating the local overhanging property, which has recently been required as a property that a cold-rolled steel sheet should have, and which can clearly discriminate the property difference between different materials. Placed in offering.

<Means for Solving the Problems> The inventors of the present invention have conducted a number of experiments in order to achieve the above-mentioned object, and as a result of the research, “the contact portion with the steel sheet can be kept extremely small in the initial stage of forming. Using a cone-shaped mold (punch) with a relatively sharp tip for the extension molding that completely restrains the flow of material under the application of a lubricant with a relatively low viscosity (complete extension molding) , It is possible to accurately evaluate the local overhanging property of the steel sheet that corresponds to the actual situation. "
It came to the knowledge that.

The present invention has been made on the basis of the above findings and the like. “As shown in FIG. 1, the radius of curvature (R) of the punch tip is 1.0 to 2.0 mm, the apex angle of the punch (θ) is 90 to 110 degrees. Using the conical punch 1 of No. 1 to perform full overhang forming of the steel plate 2 under the use of lubricating oil of No. 1 and obtain the formable height (H: Forming height immediately after breaking), and accurately evaluate the local overhang It is characterized in that it is possible to do. In FIG. 1, reference numeral 3 is a die, and 4 is a plate retainer.

That is, the present invention uses the punch 1 having a specific size and the die 3
Steel plate 2 that is fixed so that it does not flow in by means of plate and plate retainer 4
Is formed by bulging under lubrication with lubricating oil (generally commercially available is sufficient), and the forming height (H) immediately after fracture
In the following, the local overhanging property of the material is evaluated, but the reason why the size of the punch is limited as described above in the present invention will be described in more detail together with its action.

<Operation> First, in the test method according to the present invention, the radius of curvature (R) of the tip of the cone punch to be used is set to 1.0 to 2.0 because the radius of curvature (R) exceeds 2.0 mm in the initial stage of molding. In addition to the fact that the material does not fit into the tip of the punch and local overhang deformation does not occur, therefore the local overhang property cannot be evaluated, and the difference between different materials does not appear as in the case of normal overhang molding. Yes, while the radius of curvature (R) is 1.
If it is less than 0 mm, the steel sheet as the test material will be perforated and it will not be possible to perform stretch forming.

When the apex angle (θ) of the conical punch is less than 90 °, the steel plate is perforated. On the contrary, when the apex angle (θ) exceeds 110 °, contact between the punch and the material at the initial stage of forming. Since the area increases relatively and the local overhanging property cannot be evaluated,
The cone punch apex angle (θ) was limited to 90 to 110 °.

In the bulging test of the steel sheet, the influence of the lubricant is generally very large, but in the test method according to the present invention, the influence of the lubricant is relatively small because the initial contact area is small. for that reason,
Any commercially available lubricating oil can be stably molded. However, if molding is performed without lubrication, the local elongation is suppressed by the frictional force between the material and the tool, and it becomes difficult to evaluate the material difference.

In this case, if molding is performed in a highly lubricated state where a commercially available lubricating oil and a solid lubricant (such as a vinyl sheet) are used together, distortion will be extremely concentrated at the tip of the punch, and the punch apex angle will be small. As in the case of the above, the material is perforated and the overmolding cannot be performed.

Further, in the test method according to the present invention, it is desirable that the molding speed is relatively low in order to detect the presence or absence of breakage, and if possible, it is good to carry out at an extremely low speed of 0.1 to 0.5 mm / sec.

Next, the present invention will be described more specifically by way of examples.

<Example> First, four types of cold-rolled steel sheets having a chemical composition and properties shown in Table 1 and having a thickness of 0.8 mm were prepared, and for each of them, a local overhanging test method according to the present invention and a conventional Erichsen test, A CCV test and a hydraulic bulge test were conducted, and the results are shown in Table 2.

In the local overhang test method according to the present invention, a conical punch having a radius of curvature (R) of the tip: 1.3 mm, an apex angle (θ): 90 °, 95 ° and 100 ° (3 types), a diameter: 50 mm, And, the effect of temperature is small as a lubricant A white white valice was used.

As is clear from the results shown in Table 2, the measured values in each of the conventionally used formability test methods were almost the same for all four types of steel sheets, and there was no difference in formability between them. On the other hand, according to the test method of the present invention, the difference in local overhang formability of each material can be clearly determined, and the recent severe required performance required for cold-rolled steel sheet can be accurately evaluated. I understand.

That is, the test according to the present invention is a method for evaluating the local overhanging property. Even from the above results, by applying the method of the present invention, the "sharpness of the embossed portion" and the "molding" in the overhanging molding in the actual molding work can be performed. It can be confirmed that the "limit" can be accurately evaluated, and that it can help to properly set the acceptance criterion when the punch shoulder radius is reduced during drawing.

<Summary of Effects> As described above, according to the present invention, the local overhanging property of the cold-rolled steel sheet, which could not be directly evaluated in the past, is measured by a very simple method and the material difference is clearly measured. It is possible to bring about an extremely useful effect in industry.

[Brief description of drawings]

FIG. 1 is a conceptual diagram explaining a local overhanging property test method according to the present invention. In the drawing, 1 ... cone punch, 2 ... steel plate, 3 ... die, 4 ... plate retainer.

Claims (1)

[Claims] 1. A conical punch with a radius of curvature at the tip of the punch of 1.0 to 2.0 mm and an apex angle of the punch of 90 to 110 degrees is used to evaluate the local overhanging property by fully overmolding with the use of lubricating oil. The test method for local overhang of steel sheet.