JP2000193450A - Evaluating method for press forming characteristics of steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaluate the presence of defective forming by acquiring an index from an amplitude possibility density distribution for a cross-section curve of a steel sheet surface and then comparing the index to a specified reference value which is preset. SOLUTION: A surface roughness signal for a steel sheet 1 detected with a surface roughness sensor 2 is inputted in a surface roughness measuring/calculating device 3, which is processed according to following procedure, and its result is displayed on a calculated result display device 4. Firstly, the contour of a steel sheet surface is measured over a specified length to generate a cross-section curve. Such amplitude possibility density distribution curve as a cross-section curve is equal to a cut level when a cut level of the cross-section curve from a top part to a bottom part is taken is generated. The level difference between a peak value and the top part of the distribution curve is acquired. From the ratio between the value and the maximum height of the density distribution, a press forming characteristics index is acquired. The soundness of press forming characteristics is judged by comparing it to a specified reference value which is preset. The occurrence of defective press forming is surely predicted for improve line productivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating press formability of a steel sheet such as a thin steel sheet for an automobile to be subjected to a press working.

[0002]

2. Description of the Related Art Steel sheets, for example, cold-rolled steel sheets, have been widely used for body panels of automobiles and home electric appliances, but recently, especially for body panels of automobiles, the sharpness after painting is high. There is a strong demand for the use of surface-treated steel sheets, such as hot-dip galvanized steel sheets, from the viewpoint of rust prevention, and the proportion of use of surface-treated steel sheets is rapidly increasing. Has become.

[0003] The surface properties of such a surface-treated steel sheet include, for example, a method for evaluating the amplitude level of the steel sheet surface by analyzing surface roughness, surface waviness, frequency analysis (see JIS B0601), a contact ratio, a load curve, and the like. In general, evaluation is performed by a cross-sectional shape evaluation method of a steel sheet surface using an amplitude probability distribution curve or the like. These conventional evaluation methods are all related to the workability, paintability, or finished state after painting of the steel sheet.

[0004] With regard to workability, the sliding characteristics between a steel plate and a press die pose a problem, particularly when processed into a vehicle body part by press forming. That is, if the sliding characteristics are poor, the slip between the steel plate and the press die becomes worse, and the sliding of the steel plate during pressing is suppressed, which may result in material breakage. Generally, in order to improve the slidability of a steel sheet as described above, it is necessary to control the surface properties of the steel sheet, hold press oil and rust preventive oil in the gap between the steel sheet and the mold, and use the lubrication effect thereof. Is being done.

[0005] In order to fully exert the above effects,
Conventionally, it has been said that it is effective to increase the surface roughness (Ra) of a steel sheet and to keep press oil or the like sufficiently on the steel sheet surface, and Ra is used as an evaluation parameter for the press workability of the steel sheet. That has generally been done.

[0006]

The sliding behavior between a steel plate and a press die in press working is greatly affected by the physical properties and surface properties of the steel plate surface. It largely depends on the lubricating effect of oil, cleaning oil, etc. As a method of improving the lubricating effect between the steel plate and the press die, it is generally practiced to increase the surface roughness (Ra) of the steel plate so that the oil is sufficiently held in the gap between the steel plate and the die.

However, if Ra is excessively increased, the appearance of the steel sheet after coating is impaired, and the press forming property of the steel sheet is impaired by changing the lubricating effect to a non-uniform effect. It also causes the shape to be distorted. Therefore, Ra cannot be unnecessarily large, and Ra cannot be simply used as an evaluation parameter for the press workability of a steel sheet.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an optimum method for judging the quality of press formability when press forming a steel sheet.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a method for evaluating the press formability of a steel sheet when the steel sheet is pressed by a metal mold, the method comprising: A step of creating a cross-sectional curve by measuring over the same, and a step of creating an amplitude probability density distribution from the values of each height from the highest peak to the lowest valley of the cross-sectional curve,
A step of obtaining a level value corresponding to the peak of the amplitude probability density distribution, and a press formability index from a ratio of the level value corresponding to the peak and the amplitude value between the highest peak and the lowest valley of the sectional curve. Determining the presence or absence of press formability defects by comparing the press formability index with a predetermined reference value set in advance, and determining the press formability of the steel sheet. The above problem was solved by the evaluation method.

[0010] Also provided is a method for evaluating the press formability of the steel plate, the level of the level difference to the level value and the maximum crest corresponding to the peak of the amplitude probability density distribution and H _u, corresponding to the peak The press formability index P ₀ is defined as P ₀ = {H _u / (H _u + H _d )} × 100 (%), where H _d is the difference between the value and the lowest valley bottom, and this P ₀ is preset. By comparing the predetermined reference value P _s (%) set forth above, it was found that it is preferable to evaluate press formability as good when P ₀ ≦ P _s .

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the measurement of the roughness of the steel sheet surface applied in the method for evaluating the press formability of a steel sheet according to the present invention will be described. FIG. 1 is a schematic diagram showing an example of the configuration of a surface roughness measuring device used in the present invention, wherein 1 is a steel plate as a material to be measured, 2 is a laser beam type surface roughness sensor, for example, and 3 is a surface roughness. The measurement calculation device 4 is a calculation result display device. The surface roughness signal of the steel sheet 1 detected by the surface roughness sensor 2 is input to the surface roughness measurement and calculation device 3 and processed by the surface roughness measurement and calculation device 3 in the following procedure. The result is displayed on the calculation result display device 4. First, the surface shape of the steel sheet surface is measured over a predetermined length L (mm), and a cross-sectional curve A as shown in FIG. In the section curve A, when the cutting level from the highest peak P to the lowest valley bottom V is taken, the probability that the section curve becomes equal to the cutting level as shown in FIG. It is created as a distribution curve B. A cutting level difference _Hu between the peak value C of the amplitude probability density distribution curve B and the highest peak P is obtained. From the ratio between the cutting level difference _Hu and the maximum height _Rmax (= _Hu + _Hd ) of the amplitude probability density distribution, the press formability index P
₀ is obtained by the following equation (1).

P ₀ = {H _u / (H _u + H _d )} × 100 (%) (1) The press formability index P ₀ is compared with a predetermined reference value P _{s which} is set in advance, and the press is performed. The formability is determined. Here, supplementing the above-mentioned press formability index P ₀ , first, that the press formability index P ₀ is small means that the peak value C of the amplitude probability density distribution curve B is small as shown in FIG. Since the cutting level is high, FIG.
As shown in FIG. 5, a deep valley and a plateau-like topography are shown, and a small peak M in this plateau is easily worn away, and the surface in contact with the mold becomes a smooth flat surface, which easily causes slippage. This tendency is remarkable especially on a soft surface such as a hot-dip galvanized steel sheet.

On the other hand, a large press formability index P ₀ means that the cutting level of the peak value C is low as shown in FIG. 4 (a), and therefore, as shown in FIG. 4 (b). It represents a terrain having a steep, high mountain shape, and the high peak N in this mountain does not disappear easily even in contact with the mold, and the frictional force is maintained, so that the press formability deteriorates. Also, increasing the press formability index P ₀ is generally becomes possible to enhance the roughening processing costs, and so also adversely affect the finished quality of the painted surface, P ₀ necessary to fit the press-molding conditions Is preferably provided.

The Stribeck curve shown in FIG. 7 generally describes the lubricating state of the steel plate and the mold as described above, and is a curve that is often used for distinguishing the lubricating state. R in FIG.
_{The first} region is called a fluid lubrication state, in which no contact between metals occurs, and the frictional force depends only on the viscous resistance (η) of the lubricating oil forming the fluid film. The R ₂ region is called a boundary lubrication state, in which the two surfaces of the steel plate and the mold are separated by a thin boundary lubrication film of several molecules or less, and exhibit a Coulomb friction characteristic state in which metal contact occurs locally. .

The R ₃ region is called a mixed lubrication state, and is a mixed state of the above R ₁ and R ₂ regions. Part of the vertical load is a fluid lubricating film, and the rest is a convex portion on the surface of the steel plate for boundary lubrication. It is in a state of being supported through. In the form of lubrication in press working, since the surface pressure is generally higher and the speed is slower than the sliding surface of the machine element, a complete fluid lubricating film is formed over the entire sliding surface of the steel plate and the mold. Case (R
₁ area) is almost nonexistent.

On the other hand, all the sliding interfaces are in a boundary lubrication state (R
That almost without comprising ₂ regions), mixed lubrication state is these intermediate states in pressing (R ₃ region)
In most cases, processing is performed. FIG. 8 schematically shows a state in which two surfaces of the steel plate 1 and the mold 5 are in contact with each other and sliding in the press working. In FIG. 8, a portion where the convex portion on the surface of the steel plate 1 as the workpiece is restrained by the mold 5 and flattened is shown as a. The concave portion on the surface of the steel sheet 1 is called a micro pool 7, and the lubricating oil 6 is sealed therein. As described above, when the micro pool 7 is sufficiently filled with the lubricating oil, a static pressure is generated by the lubricating oil, and a part of the working surface pressure Pr of the press working is shared by the static pressure. The force F decreases.

As described above, the press formability in press working can be evaluated by associating the state of irregularities on the surface of the steel plate in press working with the above-mentioned lubrication mechanism. Sex index P ₀
Can be confirmed. That is, as P ₀ is smaller (that is, as the height of the projections is lower), the projections on the surface of the steel sheet are more easily flattened by the mold, and the ratio of receiving the working surface pressure in the micro pool increases, and the mixed lubrication state is increased. (R ₃ area)
And the frictional force decreases.

Conversely, the larger the _value of P ₀ (ie, the higher the height of the projections), the more difficult it is for the projections on the steel sheet surface to be flattened by the mold. small becomes, the frictional force is increased the shape of a boundary lubrication state (R ₂ region), it is to be considered to become poor press formability.

[0019]

EXAMPLE As an example of the present invention, the results of confirming the press formability of a hot-dip galvanized steel sheet by actually pressing automotive parts using the hot-dip galvanized steel sheet will be described. The hot-dip galvanized steel sheet used here is a GA material with a thickness of 0.8 mm and a width of 1585 mm. A total of 12 coils of this hot-dip galvanized steel sheet were prepared.

A sample was taken of each coil, and a measurement calculation of a press formability index P ₀ was performed using the method for evaluating press formability of a steel sheet of the present invention. Then, after that, each coil was pressed to produce an automobile part, and the actual press formability was evaluated. Table 1 shows the results.

For comparison, a conventional three-dimensional surface roughness was used.
The SRa value and the tensile property value were measured simultaneously, and the results are shown in Table 1.

[0022]

[Table 1]

Here, the quality of press formability was determined according to the following criteria. ：: No cracking occurred even after pressing 500 sheets. Δ: Cracks occur in 201 to 499 presses. ×: Cracks occurred in a press of 200 sheets or less, and the press work was stopped. Here, as typical examples in Table 1, coil Nos. 2, 8,
Excerpts from the four cases 9 and 11, the amplitude probability density distributions and the press formability index P calculated from the distributions
₀ is shown in FIG.

FIG. 6 is a graph showing the press formability index P ₀ and the press formability. As is clear from Table 1 and FIG. 6, the tensile properties of the steel sheet (YS, TS, EI)
Press formability cannot be properly evaluated from the surface roughness (SRa). However, by using the press formability index P ₀ of the present invention, it is possible to determine whether the press formability is good or not.

[0025] In the case of FIG. 6 is a present embodiment is preferably set to 32% a predetermined reference value P _s, is P ₀
If it is 32% or less, it can be determined that the press formability is good.

[0026]

As described above, according to the present invention,
It is possible to make a highly accurate determination on the press working, and it is possible to reliably predict whether or not a press failure has occurred, thereby greatly contributing to an improvement in the productivity of the press working line.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of a configuration of a surface roughness measuring device used in the present invention.

FIGS. 2A and 2B are characteristic diagrams of (a) a sectional curve and (b) an amplitude probability density distribution curve.

FIG. 3 is an explanatory diagram of (a) an amplitude probability density distribution curve and (b) a cross-sectional curve when the press formability index P ₀ is small.

FIG. 4 is an explanatory diagram of (a) an amplitude probability density distribution curve and (b) a cross-sectional curve when the press formability index P ₀ is large.

FIG. 5: Amplitude probability density distribution curve and press formability index P ₀
Is a graph illustrating some of the relationships.

FIG. 6 is a graph showing a value of a press formability index P _{0 in} an example.

FIG. 7 is a Stribeck curve schematically showing a form of lubrication.

FIG. 8 is a schematic diagram illustrating contact between a steel plate and a mold.

[Explanation of symbols]

Reference Signs List 1 steel plate (workpiece material) 2 surface roughness sensor 3 surface roughness measurement calculation device 4 calculation result display device 5 mold 6 lubricating oil 7 micro pool (portion where lubricating oil is enclosed) P ₀ press formability index P _s Reference value (for press formability index)

Claims (2)

[Claims] 1. A method for evaluating press formability of a steel sheet for evaluating press formability when a steel sheet is pressed by a mold, comprising measuring a surface shape of a steel sheet surface over a predetermined length to obtain a sectional curve. The step of creating, the step of creating an amplitude probability density distribution from the values of each height between the highest peak and the lowest valley of the cross-sectional curve, and finding the level value corresponding to the peak of this amplitude probability density distribution A step of obtaining a press formability index from a ratio of a level value corresponding to this peak and an amplitude value between the highest peak and the lowest valley bottom of the cross-sectional curve; and setting the press formability index in advance. Evaluating the presence or absence of press formability defects by comparing with a predetermined reference value. 2. The method for evaluating press formability of a steel sheet according to claim 1, wherein a difference between a level value corresponding to a peak of the amplitude probability density distribution and a maximum peak is H _u , The press formability index P ₀ is defined as P ₀ = {H _u / (H _u + H _d )} × 100 (%), where H _d is the level difference between the level value corresponding to and the lowest valley bottom. _A method for evaluating press formability of a steel sheet, wherein ₀ is compared with a predetermined reference value P _s (%) set in advance, and when P ₀ ≦ P _s , the press formability is evaluated as good.