JP2002066637A - Press forming method excellent in dimensional accuracy of formed object - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hut shape bending member excellent in shape frozenness in a press component using a steel plate. SOLUTION: Using a steel plate or a coated steel plate whose ratio of crystalline plane between the crystal face of ferrite 100} in the parallel surface to the rolling surface of the steel plate and the crystal face 111} is 1.0 or more, the press forming is performed imparting a tensile stress of 40 to 100% against the material tensile strength to the longitudinal wall portion. In this case, by setting up a blank holding load F [N] so as to satisfy the formula, 2btσB/5 μ<=F<=btσB/μ, the press forming is performed imparting the tensile stress of 40 to 100% against the material tensile strength to the longitudinal wall portion. (but, b: blank width [mm], t: raw steel plate wall thickness [mm], σB: tensile strength [MPa] of raw steel plate, μ: frictional coefficient of raw steel plate and die).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to a hat bending process for obtaining a molded product excellent in dimensional accuracy by using a steel plate having a {100} texture developed for a processed part of an automobile and the like. The present invention relates to a press molding method.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 1, a hat-shaped member 1 with a flange is used very frequently as a member for structural strength of an automobile such as a member, and a flange surface is used as a spot welded portion when assembling a vehicle body. It is used. Such a hat-shaped member may be manufactured by bending a steel sheet into a hat shape by press-forming using a press-forming mold including a punch 4, a die 3, and a blank holder 2 as shown in FIG. 2, for example. Many. Recently, in order to suppress the amount of carbon dioxide gas emitted from automobiles, application of a construction method for reducing the weight of automobile bodies by using high-strength steel plates has been promoted. In addition, in order to ensure the safety of the occupants, high-strength steel sheets in addition to mild steel sheets are increasingly used for structural strength members such as members and parts of automobiles.

[0003] In order to further reduce the weight of automobile bodies in the future, new demands for further increasing the use strength level of high-strength steel sheets are increasing very much. However, when bending is applied to a high-strength steel sheet,
Due to its high strength, the shape after processing tends to return to the shape before processing away from the shape of the processing jig. This phenomenon of returning to the original shape direction even after processing is called springback. When this springback occurs, it is difficult to join the members by spot welding or the like at the time of assembling the vehicle body, because the target shape of the processed part cannot be obtained.

[0004] Therefore, in the conventional automobile body, mainly high-strength steel sheets of 440 MPa or less have been used. Despite the need to reduce the weight of automobile bodies by using high-strength steel plates of 490 MPa or more, parts with low springback and excellent dimensional accuracy using high-strength steel plates of 490 MPa or more for automobile bodies The fact is that there is no method for molding the. Needless to say, it is needless to say that reducing the springback of a high-strength steel sheet or a mild steel sheet of 440 MPa or less is extremely necessary for enhancing the shape accuracy of products such as automobiles and home electric appliances. {100} invented by the present inventors
A ferritic thin steel sheet having an improved texture and excellent in bending workability is disclosed in WO00 / 06791.
However, a method of forming a hat-shaped member that causes wall warpage in a vertical wall portion is not disclosed in WO 00/06792.

[0005]

When hat bending is applied to a mild steel sheet or a high-strength steel sheet, large springback occurs depending on the strength of the steel sheet, and the shape-freezing property of the machined part is poor at present. is there. The present invention fundamentally solves this problem and provides a press molding method excellent in dimensional accuracy of a molded product.

[0006]

According to the conventional knowledge, it has been considered that it is important to reduce the yield point of a steel sheet as a measure for suppressing springback. In order to lower the yield point, a steel sheet having a low tensile strength had to be used. However, this alone is not a fundamental solution for improving the hat bending workability of the steel sheet and suppressing the amount of springback.

Therefore, the present inventors newly focused on the influence of the texture of the steel sheet on the bending workability in order to improve the hat bending workability and fundamentally solve the occurrence of springback. The effect was investigated and studied in detail. The intent was to find a good material index for hat bending workability of steel sheets and forming conditions. As a result, in the texture of the steel sheet, the {100} crystal plane parallel to the sheet surface and the {1}
Using a steel plate having a 11｝ crystal plane ratio of 1.0 or more,
It has been clarified that, when punching is performed while applying a tensile stress of 40 to 100% of the material tensile strength to the vertical wall portion, a hat-shaped member having excellent dimensional accuracy can be obtained. The present invention is configured based on the above findings. The main points are as follows.

(1) In the press forming method of a steel sheet, {100} of ferrite on a plane parallel to a rolling surface is provided.
Using a steel plate or a plated steel plate in which the ratio of the crystal plane to the {111} crystal plane is 1.0 or more, the vertical wall portion has a material tensile strength of 40%.
A press molding method excellent in dimensional accuracy of a molded product, wherein the molding is performed while applying a tensile stress of １００100%. (2) The press molding method according to the above (1), wherein the wrinkle pressing load F is set in a range satisfying the following formula (1), and the molded product has excellent dimensional accuracy. _{2btσ B / 5μ ≦ F ≦ btσ} B / μ ... (1) provided that b: blank Width [mm], t: material plate thickness [mm],
σ _B : Tensile strength of base plate [MPa], μ: Friction coefficient between base plate and mold

(3) The molded article according to the above (1) or (2), wherein a protrusion-shaped bead is provided on the die surface in a direction perpendicular to the material inflow direction. Press molding method. (4) The press forming method according to any one of the above (1) to (3), wherein the hat-shaped member is press-worked, and the molded product has excellent dimensional accuracy.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The contents of the present invention will be described below in detail. In the press forming according to the present invention, in particular, a spring-back is generated, and a hat-type bending test with a flange, in which it is difficult to ensure shape freezing, is performed. We found that the combinations are closely related. The vertical wall tension as used herein refers to the stress applied to the vertical wall during forming as shown in FIG. 2, where P is the maximum forming load applied to the punch, b is the width of the steel sheet, and t is the thickness of the steel sheet. In this case, the nominal vertical wall tension σ _t is given by equation (2). σ _t = P / 2bt (2) where σ _t : nominal vertical wall tension [MPa], P: maximum forming load [N]

[0011] The basis of the present invention is that the ratio of the {100} crystal plane parallel to the plane of the thin steel sheet to the {111} crystal plane is 1.0.
If the above-mentioned steel sheet is formed while applying a tensile stress of 40% or more of the material tensile strength to the vertical wall portion, the hat bending workability of the thin steel sheet is greatly improved. The reasons for limiting the numerical values are as follows. First, the existence ratio of the {100} crystal plane and the {111} crystal plane was limited to 1.0 or more. When this ratio was smaller than 1.0, the opening amount due to elastic recovery when bending a thin steel plate. Is very large. The reason why the opening amount due to elastic recovery becomes very small when the crystal plane abundance is 1.0 or more is considered to be because plastic deformation in the steel sheet during bending is extremely smooth. Considering bending deformation from the viewpoint of crystallography, it is considered that the fact that there are many {100} crystal planes means that bending deformation proceeds only by a simple slip system.

On the other hand, if there are many {111} crystal planes, a plurality of complicated slip systems will be activated during bending.
In other words, $ 11 for bending deformation
The existence of the 1｝ crystal plane seems to be inconvenient.
From these facts, it can be seen that the abundance of the {100} crystal plane is {11}
It can be understood that when the ratio becomes larger than that of the 1｝ crystal plane and the ratio becomes 1.0 or more, the bending deformation smoothly proceeds. The amount of crystal planes parallel to the plane of the thin steel sheet is assumed to be proportional to the amount of X-ray diffraction,
It is determined by measuring the diffraction intensity of X-rays such as {200} and {222}.

Therefore, X such as {200} or {222}
The diffraction intensity of the line means that the corresponding crystal planes as entities correspond to the abundance of {100} and {111} crystal planes, respectively. Needless to say, the X-ray diffraction intensity ratio,
{200} / {222} is the ratio of existing crystal planes {10
It can be said that it is equivalent to 0 / {111}.
The upper limit of the abundance ratio between the {100} crystal plane and the {111} crystal plane can be obtained without particular limitation, but the rank ford value is lowered and the deep drawability is deteriorated, so that the upper limit is 10 or less. Is preferred.

The reason why the tensile stress applied to the vertical wall portion is limited to 40% or more of the material tensile strength is that if this value is less than 40%, the opening amount due to elastic recovery when bending a thin steel plate. This is because the effect of reducing springback cannot be obtained. {100} crystal plane and {11}
When the tensile stress of the vertical wall of a steel plate having a crystal plane ratio of 1.0 or more is 40% or more of the material tensile strength, the opening amount due to elastic recovery becomes very small because tension is applied. It is considered that the plastic deformation of the vertical wall portion smoothly progresses, and the shape bent at the die R portion is finely bent back at the vertical wall portion. The upper limit of the tensile stress applied to the vertical wall portion is set to 100% or less of the material tensile strength in order to avoid the occurrence of fracture.

Further, as a method of applying tension to the vertical wall portion, there is a method of applying a wrinkle holding load. The wrinkle pressing load is a load applied to the blank holder by hydraulic pressure or the like as shown in FIG. 2 in order to suppress the occurrence of wrinkles during press forming. By adding this, frictional resistance is generated on the flange surface, and it is possible to generate tension on the vertical wall portion when forming with a punch. The wrinkle holding load F may be applied so that the vertical wall tension is 40 to 100% of the tensile strength of the material, but if it is set in the range of the expression (1), the springback can be sufficiently reduced. F is 2 btσ _B /
If it is smaller than 5 μm, the vertical wall tension is not sufficient, and the effect of reducing the springback cannot be obtained. Also,
If F exceeds btσ _B / μ, the tension of the vertical wall portion exceeds the breaking strength, and there is a possibility that a crack may occur.

Further, as a method of applying tension to the vertical wall portion, there is a method of providing a projecting bead on the die surface in a direction perpendicular to the material inflow direction. The bead has a protruding shape as shown in FIG. 3, and controls the inflow of the material by the bending / unbending deformation resistance and frictional resistance of the material when the material passes through the bead during molding. Also, bead 5
As a cross-sectional shape, a round cross section, a square cross section as shown in FIG.
There are steps. Since the tension of the vertical wall provided by the bead is determined by the strength, plate thickness and cross-sectional geometry of the bead, it is necessary to select a bead having an appropriate cross-sectional shape according to the material. Actually, the vertical wall tension σ _B obtained by the equation (2) is 40 to 100% of the material tensile strength.
It is sufficient to set a bead shape such that

The thin steel sheet to be used in the present invention has a thickness of 6 mm.
The following steel sheet is defined, but what is important here is that, in all the thin steel sheets from low strength steel sheet to high strength steel sheet, the {100} crystal plane and the {111}存在 If the abundance ratio of the crystal plane is 1.0 or more and the punch forming is performed while applying a tensile stress of 40 to 100% of the material tensile strength to the vertical wall portion, the hat bending workability of the thin steel sheet is very high. Is to improve. In other words, it is a basic processing method related to press deformation that exceeds the restriction on the strength level of the thin steel sheet.

The above concept can be universally applied to a thin steel sheet, so that it is basically unnecessary to limit the type of the thin steel sheet. However, from a practical point of view,
As an application example of this technology, the type of thin steel sheet will be mentioned. The types of thin steel sheets range from mild steel sheets to high strength steel sheets. And, of course, the distinction between the hot-rolled steel sheet and the cold-rolled steel sheet does not matter at all. The effect of the present invention is that a steel plate having an existing ratio of {100} crystal plane and {111} crystal plane parallel to the plane of the thin steel sheet is 1.0 or more, and the vertical wall portion has 40 to 100% of the material tensile strength. It is obtained by performing punch forming while applying a tensile stress of the above, but in order to obtain a more remarkable effect, the existence ratio is 1.5 or more, or the tensile stress is 60 to 100%. Is preferred. The punch shoulder radius and die shoulder radius are usually used in a range of 2 to 15 times the plate thickness, but are not particularly specified here.

The clearance between the punch and the die is usually used in the range of 0.8 to 3 times the plate thickness, but is not particularly specified here. Further, the present invention is also applicable to a steel sheet obtained by plating the above-mentioned thin steel sheet, and the type of plating is not particularly limited, and the effect of the present invention can be obtained by any of electroplating, hot-dip plating, and vapor deposition plating. Can be The press forming method according to the present invention can be applied not only to bending, but also to composite forming mainly involving bending, such as overhang, drawing, and stretch flange.

[0020]

EXAMPLES The composition of the components is C: 0.09% in mass%, Si:
0.02%, Mn: 1.86%, P: 0.017%,
S: 0.017%, Al: 0.055%, N: 0.00
The result of the examination using carbon steel having 35% will be described. These steels have a slab heating temperature of 900
It heated between the temperature of 1250 degreeC, and performed hot rolling, and finished it to the steel plate of thickness 3mm. Thereafter, the sheet thickness was reduced to 1.4 mm by cold rolling, and the steel sheet was subsequently annealed by a continuous annealing method at a soaking temperature of 700 ° C to 850 ° C. Table 1 shows mechanical property values and X-ray diffraction intensity ratios including the hot-rolled steel sheet hot-rolled to 1.4 mm, which is the same as the cold-rolled steel sheet obtained by the above-described method. The evaluation of the springback was performed by performing a hat type molding test as shown in FIG.
The obtained molded product was evaluated.

As an experimental condition, the blank shape is 270 ×
50mm, tool punch width 78mm, punch shoulder radius 5m
m, die shoulder radius 5 mm, and clearance 1.6 mm. The test was performed by changing the ratio of the vertical wall tension to the material tensile strength to 20 to 120% by changing the forming height to 70 mm and changing the wrinkle holding load. The vertical wall tension σ _B is obtained by measuring the punch forming load P,
It was calculated using the equation. The dimensional accuracy of the molded product was evaluated by measuring the amount of wall warpage (vertical wall curvature) 1 / ρ opening angle Δθ = θ-90 degrees (the angle between the punch bottom and the vertical wall) by measuring the three-dimensional shape. Performed and compared. The evaluation results are also shown in Table 1.

[0022]

[Table 1]

With steel types A and D, even if the vertical wall tension was changed, it was not possible to obtain a molded product having a wall warpage of 4.0 × 10 ⁻³ (1 / mm) or less and an opening angle of 4 ° or less. In addition, steel types B and C
And when the vertical wall tension is 20%, the wall warp is 5.0 × 10 ^-3 (1 /
mm) or less, a molded product having an opening angle of 5 degrees or less cannot be obtained.
Sufficient dimensional accuracy was not obtained. On the other hand, when the vertical wall tension of steel types B and C is 40% and 60%, the wall warp is 3.0 × 10 ⁻³ (1 / m).
m) or less, the opening angle was 3 degrees or less, the dimensional accuracy was greatly improved, and good products could be obtained. Further, when the vertical wall tension exceeded 100% of the material tensile strength, the material was stretched and broken at the vertical wall portion, and a molded product could not be obtained. The steel plate having the ratio of {100} crystal plane and {111} crystal plane of ferrite in the plane parallel to the rolling plane of the present invention of 1.0 or more is used, and the vertical wall portion has a material tensile strength of 40 to 1.
By the press forming method of performing punch forming while applying a tensile stress of 00%, it is possible to efficiently and efficiently process a hat-shaped member.

[0024]

According to the present invention, when a material having a controlled texture of a thin steel sheet is formed while applying tension to a vertical wall, its hat bending workability is remarkably improved, the amount of springback is small, and It has become possible to provide a hat-shaped member mainly formed by bending and having excellent shape freezing properties.
In particular, a high-strength steel sheet can be used for a part in which a high-strength steel sheet has conventionally been difficult to be applied due to a problem of poor shape.
The use of high-strength steel sheets is indispensable for promoting the weight reduction of automobiles. If a hat-shaped member using a high-strength steel plate and having a small springback can be formed, it can contribute to further weight reduction of automobiles. Therefore, the present invention is an industrially extremely valuable invention.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a hat-shaped bent product.

FIG. 2 is a cross-sectional view showing vertical wall tension in hat-shaped bending.

FIG. 3 is a diagram showing an example of a bead positional relationship.

FIG. 4 is a diagram showing an example of a cross-sectional shape of a bead.

FIG. 5 is a sectional view showing the amount of wall warpage of a hat-shaped bent product.

FIG. 6 is a cross-sectional view showing an opening angle of a hat bent product.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Hat-type member with flange 2 Blank holder 3 Die 4 Punch 5 Bead

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Manabu Takahashi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Naoki Yoshinaga 20-1 Shintomi, Futtsu-shi, Chiba Made in New Japan F-term in the Technology Development Division of Steel Corporation (reference) 4E063 AA01 BA01 CA06 DA01 JA01 JA07 KA06 LA12 MA18

Claims (4)

[Claims] In a method for press-forming a steel sheet, a steel sheet or a plated steel sheet having a ratio of a {100} crystal plane and a {111} crystal plane of ferrite in a plane parallel to a rolling plane is 1.0 or more, 40 to 10 of material tensile strength on vertical wall
A press molding method excellent in dimensional accuracy of a molded product, wherein molding is performed while applying a tensile stress of 0%. 2. The press molding method according to claim 1, wherein the wrinkle pressing load F [N] is set in a range satisfying the following equation (1). _{2btσ B / 5μ ≦ F ≦ btσ} B / μ ... (1) provided that b: blank Width [mm], t: material plate thickness [mm],
σ _B : Tensile strength of base plate [MPa], μ: Friction coefficient between base plate and mold 3. The press-forming method according to claim 1, wherein a protrusion-shaped bead is provided on the die surface in a direction perpendicular to the material inflow direction. 4. The press-forming method according to claim 1, wherein the hat-shaped member is press-worked.