JPH049225A - Press-forming method - Google Patents

Abstract

PURPOSE:To improve the dimensional accuracy in press forming by specifying the forming load of a steel sheet having specified yield strength. CONSTITUTION:The steel sheet having 4% yield stress when deformation is stopped as stress is given is press-formed on condition that a forming load given to the bottom dead center is retained for 1sec. In this way, as fixed yield strength is secured, press-forming can be executed in a drastically excellent dimensional accuracy and this method can correspond to high quality and high accuracy of the press formed part. This press-forming method is not restricted to a large area part such as a car body panel but useful for press-forming all parts requiring high dimensional accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for press forming a steel plate, particularly a press forming method with good dimensional accuracy after forming, such as good shape fixability.

(Prior Art) For large-area parts whose appearance is important, such as automobile body panels, it is extremely important to maintain the accuracy of the surface shape during the press forming process. In such applications, in order to avoid defects in dimensional accuracy such as spring banks and surface distortion, it is effective to lower the yield strength of the steel plate to be formed. 0,06
Low-carbon killed steel sheets containing about 10% by weight of carbon are widely used. Furthermore, recently, in addition to conventional deep-drawing steel plates, ultra-deep-drawing steel plates with lower carbon content and lower yield strength have been manufactured.

However, in recent years, due to design and functional requirements, there has been a growing trend for automotive molded parts to have good dimensional accuracy, and even lower yield strength. .

Although the use of plastic for such parts is being considered, there are many problems such as cost and maintenance, and this has not been realized.

(Issues to be solved by the invention!) As mentioned above, there is a need to lower the yield strength of steel plates for press forming, but simply lowering the yield strength does not allow the molded product to The strength of the material decreases, which poses a practical problem.

Defects in dimensional accuracy such as spring banks and surface distortion mainly depend on the yield strength of the steel plate, and the lower the yield strength, the smaller these defects become. In conventional steel sheets for ultra-deep drawing, this bending point is approximately 11 to 13 kgf/am.
However, it still cannot meet the recent strict demands for dimensional accuracy. However, if we were to lower this value further, apart from the specific method of achieving it,
It is expected that the strength of the parts will become too low, causing a practical problem.

Here, the key point of the present invention is to propose a press forming method that effectively utilizes a steel plate having the above characteristics.

(Means for Solving the Problem) Therefore, in order to achieve the above-mentioned object, the present inventors reduced the effective yield strength during press forming without reducing the yield strength itself of the steel plate. By focusing on the fact that dimensional accuracy during molding and component strength after molding can be achieved at the same time if In addition to the concept of , we have found that it can be further organized by considering the decrease in yield stress under deformation.

The meaning of this can be explained from the stress-strain diagram during the tensile test as shown in FIGS. 1(a) and 1(b).

First, the stress (σ) when a material transitions from an elastic state to a plastic state is determined by the yield strength (Y
P), or as shown in Figure 1 (c), it is usually 0
．． Although it is called 2% proof stress (YS), in the case of actual press forming, of course, it undergoes a certain amount of plastic deformation and in the example shown in Figure 1 (C), 3% residual strain (e) is observed. shows. The stress that occurs when this plastic strain is applied is called yield stress, and this value essentially affects the dimensional accuracy of the press-formed product. However, in general, since yield strength and yield stress correspond well, yield strength is used for practical purposes.

By the way, as shown in Figure 2, when deformation is stopped with a tensile load applied, the tensile stress (σ) tends to decrease, and this is known as relaxidine, but as a result of the research conducted by the present inventors. It was found that phenomena such as surface strain can be sorted out by the yield stress after this reduction. In other words, the amount of decrease in yield stress and the amount of surface strain generated are correlated. below,
The reduced stress at this time is called effective yield stress. Therefore, the effective yield stress appears when deformation stops, in other words, when the strain and speed are extremely small, and the apparent yield stress in press forming is reduced. On the other hand, since the strength of a part depends on the normal yield stress, the present invention has been achieved in which both the dimensional accuracy of the molded product and the strength of the part can be achieved by skillfully combining the two.

In other words, the gist of the present invention is that the rate of decrease in yield stress when deformation stops under stress is 4.
This is a press-forming method for a steel plate, in which a press-formed product with excellent dimensional accuracy is obtained by using a copper plate with a copper plate of at least 10% and holding a forming load for 1 second or more at the bottom dead center of the press-forming.

In addition, from another aspect, the present invention uses a steel plate whose yield stress decrease rate is 4% or more when deformation is stopped in a stressed state, and The strain rate is 0.2 over at least 1% of the forming stroke.
This is a press-forming method for steel sheets, in which a press-formed product with excellent dimensional accuracy is obtained by forming at a temperature of s-' or less.

Here, the rate of decrease in yield stress refers to the rate of stress decrease with respect to the stress (σ) when the tensile displacement is stopped in the second time.

(Function) Next, the function and effect of the present invention will be explained in detail with reference to the accompanying drawings.

First, the gist of the present invention is a press forming method that can fully utilize a material with a reduced effective yield stress.

A material whose effective yield stress has been reduced in this way is a steel plate whose yield stress reduction rate is 4% or more when deformation is stopped under applied stress, and which is suitable for normal press forming. There is no particular restriction as long as it is a steel plate that can be used. For example, it is a steel plate for ultra-deep drawing with a deformation point of about 11 to 13 kgf/sm. However, in order to more efficiently reduce the effective yield stress, it is preferable to reduce the carbon content to 0.003% by weight. In addition to the following, for example, Ti and Nb
A steel plate may be manufactured by cold rolling and annealing a low carbon steel with a composite addition of .

For example, in weight%, C is 0.0015% or less, Mn is
20.03-0.60%, sol, AQ: 0.1% or less, Nb: 0.003-0.015% and/or Ti
:0.005~0.03%, NO, 0.020% or less, balance Fe and unavoidable impurities, hot rolled, annealed at 68F~950''C, cold rolled, and annealed. There are cold rolled steel sheets obtained by

In a steel plate produced by the above manufacturing method, the stress change in the steel plate is large when it undergoes deformation within the range of normal press forming, and the stress reduction rate of the steel plate after the displacement stops is 4% of the yield stress. % or more, the effective yield stress is reduced and defects in dimensional accuracy such as springback and surface distortion are suppressed.

There is no particular restriction on the time it takes for the rate of decrease in yield stress to reach 4% or more after the displacement stops, but the rate of decrease in yield stress usually saturates after 30 seconds or more, so the rate of decrease should be set at 30 seconds. However, normally it is sufficient to determine the rate of decrease after one second.

Here, the reason why the stress reduction rate of the tA plate after the displacement stopped was set to be 4% or more of the normal yield strength is as follows.

In other words, as mentioned above, when the displacement of any steel plate is stopped during deformation, the stress applied to the steel plate gradually decreases, as shown in Figure 2, and the effective yield stress tends to decrease. It is in.

However, in order to suppress defects in dimensional accuracy, as is the object of the present invention, the effective yield stress must be reduced by a certain percentage or more.

According to various experiments conducted by the present inventors, even with steel plates normally used for drawing, a decrease in yield stress of about 3% of the normal yield strength is observed after a certain period of time has passed after the displacement has stopped. In this case, there is almost no effect of suppressing defective phenomena in terms of dimensional accuracy. On the other hand, the suppression effect only appears when the rate of decrease reaches 4% or more.

For the above reasons, in the present invention, the stress reduction rate of the steel plate after the displacement stops is set to 4% of the normal yield strength.
The above shall apply.

As already mentioned, the steel plate to be processed in the present invention may be tested in advance for such characteristics, and a steel plate that satisfies the requirements of the present invention may be used. It is not possible to adjust the stress reduction rate during pressing.

According to the present invention, the following two types of press forming are performed on the steel plate thus prepared.

First press forming method When press forming using the above-mentioned steel plate, if press forming is performed by a normal method, a molded product with good dimensional accuracy cannot be obtained because the yield strength itself is not much different from that of a conventional steel plate. However, according to the present invention, after the displacement is stopped during press forming, the load is maintained at the bottom dead center of the press forming for a predetermined period of time, for example, 1 second or more.
By reducing the effective yield stress (this is referred to as "determining"), the dimensional accuracy of the resulting molded product can be improved. Although there is no particular upper limit to the above-mentioned holding time, it is best to keep it as short as possible from the viewpoint of productivity, for example, 1 to 3 seconds may be sufficient.

Alternatively, the same effect can be obtained by significantly reducing the forming speed near the bottom dead center of the press forming stroke, instead of completely stopping and maintaining the displacement at the bottom dead center of the press. It will be done. In this case, the deformation speed of the steel plate during press forming naturally depends on the forming speed of the press machine, but in reality, the strain rate differs depending on the shape of the molded product, so dimensional accuracy is required. All you have to do is focus on the strain rate of the part. Further, although it is sufficient to reduce the strain rate before the bottom dead center, it is of course not preferable to widen the range where the forming speed is slow in consideration of press efficiency. On the other hand, if the range is narrow, the effective yield stress of the steel plate cannot be lowered completely.Therefore, considering the reduction state of the effective yield stress, strain rate, forming stroke, press efficiency, etc., it is necessary to reduce the effective yield stress of the steel plate by at least 1% of the total forming stroke. It is necessary to mold the range at a strain rate of 0.2 s-' or less in areas requiring dimensional accuracy.

In a range shorter than this stroke range, it is necessary to further reduce the strain rate in order to sufficiently reduce the effective yield stress, which is not practical from the standpoint of press efficiency. Further, at a molding speed higher than this strain rate, the effective yield stress does not decrease sufficiently due to the relationship with the normal molding speed, and therefore the original objective of improving dimensional accuracy cannot be observed.

Note that since the normal press forming speed is about 10' to 10''S-', it can be said that it is quite slow in the present invention.

Such a slow molding speed can be easily achieved by adjusting the pressurizing speed of the hydraulic mechanism.

Here, the strain rate is defined as follows.

In press forming of a steel plate, the steel plate generally undergoes biaxial deformation within the plane. In order to uniformly express the strain state at this time, equivalent strain in plastic mechanics is used. In other words, the maximum principal strain in the plane is ε1, and the principal strain perpendicular to it is ε.
7. If the plate thickness strain is ε1, the equivalent strain is a, and the in-plane average r value of the plate is r, then 3r(1÷2r)2.

Thus, according to the present invention, a press-formed product with far superior dimensional accuracy compared to conventional press-formed products can be obtained by both the first and second press-forming methods.

Next, the present invention will be described in more detail with reference to Examples.

Example In this example, three types of steel shown in Table 1 were used as test materials. The mechanical properties of each sample material are summarized in Table 2. The steel type targeted by the present invention is No. 1.2, and Sho 3 is a comparison material. As will be described later, the rate of decrease in yield stress at the time of stopping deformation during this comparison was less than 4%.

Table 1 Table 2 As a method for measuring effective yield stress, in this example, displacement is stopped with a fixed amount of tensile strain of 1%, 3%, and 5% applied in a normal uniaxial tensile test, and then the We investigated changes in load, that is, changes in stress.

The results are shown graphically in Figure 3 ((capital), (b)).

As shown in FIG. 3(a), the stress reduction of the test material Nα1.2 after 1 second was 4% or more. Also, looking at the data after 5 seconds, as shown in Figure 3 (b), the stress reduction rate for sample material 1.2 was 4% or more, whereas for sample material N113, the stress reduction rate was 1 second. It can be seen that not much has changed since then.

Next, in the press forming test of this example, which investigated the spring bank when each of the above test materials with dimensions of 30 x 210 + llI was formed into a U shape using a press machine, the effective yield point was In order to clarify the effect of reducing the pressure, press forming tests were conducted in two cases in which the press was stopped for 1 second at the bottom dead center. The press loads were 1 ton and 3 ton.

The results are shown graphically in FIG. 4(a), υ), (C1).

Figure 4 (→ shows the case of normal molding, but even in this case, it can be seen that the springback of sample attachment 1.2, which is the subject of the present invention, is small.Furthermore, the state where the load is applied at the bottom dead center It can be seen that the springback after performing the (determined pressing state) is smaller for the sample material kl,2, as shown in Fig. 4 et al. As described above, the steel plate that is the object of the present invention has a small springback and is excellent in shape fixability, as the springback becomes even smaller when pressed.

Furthermore, although Fig. 4 (C) shows the difference between Fig. 4 (river) and Fig. 4 (b), it is clearly recognized that according to the present invention, the amount of reduction in springback is also large. .

Further, each of the above test materials was press-molded using a hydraulic press using an actual car door model mold as shown in FIG. In such a panel in which a portion (embossed portion) that is pushed out in the opposite direction exists on the panel surface, surface distortion, which is a subtle waviness of the surface, often exists around the embossed portion.

The effect of the present invention on this surface distortion will be described below.

The molding conditions were such that the load was maintained at the bottom dead center of the press for 2 seconds, and then molding was completed.

Figure 6 shows the cross-sectional shape of the area where surface strain occurs in the molded product for sample N11.
These are the results of comparative measurements on floors 1, 3, and 2.3.

While sample material N113, which is a comparison material, has clear surface distortion, sample material No. 1, which is the subject of the present invention,
．． It can be seen that in No. 2, no surface distortion occurs and the shape is smooth. As described above, a significant effect on surface distortion at the bottom of the emboss is observed.

Next, the results of the investigation on conformity to the mold (shape fixability) at the same measurement location are the results of the seventh investigation. The numbers on the vertical axis represent the thin handle shape H (see the explanatory diagram in the figure) at 3001 spans. Originally, this handle thin shape H is about 0.38 mm if it follows the shape of the mold. In FIG. 7, the data indicated by white circles are the results of normal molding, and it can be seen that the lower the normal yield strength, the better the shape. Furthermore, by carrying out the molding method of the present invention, as shown by the data in black circles, the sample material No.
1.2 has a better shape. In other words, it can be said that the dimensional accuracy is improved not only in the surface distortion of the lower part of the embossing but also in the overall shape.

Furthermore, when press forming was carried out for Test Attachment 1 while varying the strain rate within a range of 1% before the bottom dead center of press forming, the relationship between the strain rate (ε) and the thin handle shape H was found. was as shown in Figure 8. Significant improvement is seen when the strain rate is 0.2S-' or less. Note that normal forming is done by this hydraulic press, and mechanical presses at the actual production level are faster, and the thin shape of the handle is larger.
(shape deteriorates).

(Effect of the invention) As explained above, according to the present invention, 11 to 13
While securing the yield strength of kgf/mm', the press forming of the steel plate can be performed with significantly better dimensional accuracy than the press forming of the steel plate using the conventional method, resulting in higher quality press-formed products.
It is extremely useful industrially because it can handle higher precision.

Furthermore, it is clear from the previous explanation that the press forming according to the present invention is not limited to large-area parts such as automobile body panels, but is useful for press forming general parts that require high dimensional accuracy. Businesses will understand.

[Brief explanation of drawings]

Figure 1 (a), Figure 1 [present]) and Figure 1 (C) are
Explanation of the collapse strength, 0.2% yield strength, and yield stress, respectively: Figure 2 is an illustration of the effective yield stress: Figure 3 (a) is the stress reduction rate after 1 second in the example Figure 3 (b) is an explanatory diagram of the stress reduction rate after 5 seconds in the example; Figure 4 (a) is an explanatory diagram of springback in normal molding in the example; Figure (b) is an explanatory diagram of the springback caused by holding the bottom dead center in the example. Figure 4 (C) is an explanatory diagram of the reduction rate of springback due to the difference in the molding method in the example; , a schematic explanatory diagram of the door model mold used in the examples; FIG. 6 is an explanatory diagram of an example of measuring the cross-sectional shape of a molded product; FIG. 7 is an explanatory diagram showing the measurement results of the conformability of the molded product to the mold. figure;
and FIG. 8 is an explanatory diagram showing the relationship between strain rate and mold conformability.

Claims (2)

[Claims] (1) Use a steel plate whose yield stress decrease rate is 4% or more when deformation stops under applied stress, and hold the forming load for more than 1 second at the bottom dead center of press forming. A press forming method for steel plates that produces press-formed products with excellent dimensional accuracy. (2) Using a steel plate whose yield stress decrease rate is 4% or more when deformation stops under stress, at least 1% of the total forming stroke before the bottom dead center of press forming.
A method for press forming a steel plate, in which a press formed product with excellent dimensional accuracy is obtained by forming the above range at a strain rate of 0.2 s^-^1 or less.