JPH07275937A - Bending method for clad high-strength steel sheet superior in form freezing characteristic - Google Patents

Abstract

PURPOSE:To provide a bending method, for a clad highstrength steel sheet, which is provided with a superior form freezing characteristic and which is suitable for reducing warp for the bend formed parts in automobile components. CONSTITUTION:In the method of forming into a hat-shaped cross section a clod high-strength steel sheet 6 having a high tension for the surface 1 and a mild steel for the internal layer by means of a bending tool with a U-shaped cross-section, a solid lubricant 8 is applied for 3mum or less on one side or both sides of a steel sheet 6 to be worked, and a clearance 7 is set on 80 to 100% of the thickness of the steel sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for bending a metal plate, and particularly to bending of a composite high-strength steel plate excellent in shape fixability for the purpose of preventing shape freeze defects such as warpage that occurs when a high strength steel plate is used. It relates to a processing method.

[0002]

2. Description of the Related Art Due to recent CAFE regulations in the United States and environmental protection problems, efforts have been made to reduce the weight by reducing the fuel consumption by increasing the strength of steel sheets and reducing the thickness of the steel sheets. Therefore, the inner plate and strength members for automobiles are
High-strength steel sheets exceeding 0 MPa are being used frequently, but shape freeze failure due to springback has become a problem. In particular, when a hat-shaped cross-sectional part such as a member is formed using a U-shaped bending tool, a shape freezing defect called plate warpage occurs. Therefore, application to actual parts requires a process of reworking, which is a major drawback in terms of cost and production efficiency.

Springback is caused by the distribution of residual stress generated during bending in the plate thickness direction. In the case of plate warpage, in addition to springback due to residual stress caused by bending / bending back deformation at the die shoulder, It is a complicated phenomenon that is affected by the meandering of the work material within the clearance between the punch and the die. In order to solve this problem, JP-A-56-117
In Japanese Patent No. 831, the bending radius at the die shoulder and the clearance between the punch and the die are set within a certain range based on a detailed study of the mechanism of plate warpage, and the reverse bending at the time of bending / returning at the die shoulder is controlled. It proposes a method to prevent the warp by canceling the residual moment. However, in this case, it is easily conceivable that the optimum reverse bending size strongly depends on the plate thickness and mechanical properties of the material. In fact, a high-strength steel sheet having a tensile strength of 350 MPa or more may not give a sufficient effect even within the range of the present invention, which is a problem because the effective range is limited.

[0004]

DISCLOSURE OF THE INVENTION The present invention relates to a material and a bending method for controlling a plate warp in a small range in a hat bending process without increasing the pressing step without being influenced by the mechanical properties of the material. Is.

[0005]

In a method of forming a composite high-strength steel sheet into a hat-shaped cross-sectional shape by using a bending tool having a U-shaped cross section, the steel sheet surface layer portion has a thickness of 30 from the surface layer.
% To within 500 to 1000 MPa,
The inner layer is a composite high-strength steel plate with a pressure of 250 to 400 MPa, solid lubricant is applied to one or both sides of the punch to a thickness of 3 μm or less, and the clearance between the punch and the die is 80 to 100% of the thickness of the composite steel plate. The bending method is excellent in shape fixability of the composite high-strength steel sheet.

[0006]

The present invention will be described in detail below. First, the composite steel sheet of the present invention will be described. It is generally well known that the higher the strength, the larger the spring back, but this is because the residual stress after bending is increased due to the higher yield point stress. Focusing on this point, the present invention considers a composite steel sheet in which the steel sheet surface layer portion is made of high tensile strength, the inner layer portion is softened, and the tensile strength of the entire steel sheet is the same, and the softening of the steel sheet inner layer portion is performed after bending. It has been found to be effective in reducing springback.

Next, the reasons for limiting the present composite steel sheet will be described. This steel plate is a combination of high-tensile steel on the surface layer and soft inner layer,
Also, the strength of the entire steel plate is secured by the plate thickness ratio. In the present invention, the strength of the surface layer portion is 500 to 1000.
If the pressure is less than 500 MPa, the strength of the entire steel sheet cannot be ensured, and if it exceeds 1000 MPa, the ductility is small and there is a risk of cracking from the surface layer during processing. Next, the strength of the inner layer part was set to 250 to 400 MPa was 250 M.
If it is less than Pa, the manufacturing cost becomes high and it is not practical,
On the other hand, if it exceeds 400 MPa, the strength is increased and the effect of reducing springback is reduced. Further, the reason why the plate thickness range of the surface layer high tensile strength is set within 30% is that if it exceeds 30%, the difference from the steel plate alone is small and the effect of reducing springback cannot be obtained.

Next, the reasons for limiting the bending method of the present invention will be described. Since the clearance between the punch and the die is smaller than the thickness of the steel plate to be processed, the steel plate to be processed is subjected to compressive deformation in the plate thickness direction due to ironing within the clearance. The compression in the plate thickness direction can reduce the residual stress in the plate thickness direction caused by bending and unbending at the die shoulder, and reduce the plate warp. The larger the compressive deformation at that time, the more effective it is, but the clearance of the steel sheet to be processed is 80% from the thickness of the product and the problem of the capacity load of the pressing equipment.
The range was set to -100%. In this case, if it is less than 80%, there is a concern that the plate thickness becomes too small and the rigidity and strength of the component become insufficient. On the other hand, if it exceeds 100%, sufficient plastic deformation due to compression cannot be given, so that there is no effect of reducing the plate warp.

At this time, if the solid lubricant is not applied to the steel plate to be processed, a frictional force acts between the surface of the steel plate to be processed and the tool. For this reason, the surface of the steel sheet to be processed is constrained by the surface in contact with the punch side and is compressed and deformed, and the surface on the die side is subjected to tensile deformation to generate a new residual stress distribution. Therefore,
It is necessary to apply a solid lubricant, which has excellent lubricating properties, to the work steel sheet in order to sufficiently reduce the frictional force. The solid lubricant may be applied only to the punch side surface, but it is more effective to apply it to both sides in order to make the stress distribution in the plate thickness direction uniform.

The formation of the solid lubricant will be described below.
The solid lubricant is 10 to 70 parts by weight of silica based on 100 parts by weight of the aqueous resin, a polyolefin wax having a particle size of 3 μm or less and a softening point of 120 ° C. or less and a softening point of more than 120 ° C. in a solid weight ratio. A spherical polyolefin wax having a polar group-containing molecular weight of 1000 to 4000 and an acid value of 1 to 20 containing, as a binding component, an ethylenically unsaturated carboxylic acid or its anhydride or a carboxyl group-containing derivative mixed in 9/1 to 1/9 with A solid lubricant containing a dispersion of water in an aqueous solution or an aqueous solution in a proportion of 2 to 30 parts by weight based on 100 parts by weight of the aqueous resin as a total wax solid content, or an olefin containing no volatile solvent as the aqueous resin. The above solid lubricant using acrylic resin, acrylic resin, vinyl acetate resin, or silica as diameter 5 Spherical silica sol of 0 nm, the solid lubricant using linear silica sol, or, (I) oxirane ring to be one closed one compound having one mole carboxyl group alpha, beta-ethylenically unsaturated carboxylic acid 0.8
Α obtained by reacting ˜1.1 mol and then reacting 0.5 to 1 mol of a polybasic acid and / or its anhydride.
100 parts by weight of a product (a) mainly composed of a compound having one β-ethylenically unsaturated group, and 10 if necessary
A mixture containing up to 0 parts by weight of an acrylic ester and / or a methacrylic ester (b) having one α, β-ethylenically unsaturated group, (II) with respect to 100 parts by weight of the mixture (I). 1 to 30 parts by weight of the fluororesin powder lubricant and 0.5 to 10 parts by weight of the (III) sensitizer were blended if necessary. By applying this solid lubricant to the work material, a steel sheet having extremely excellent lubricity can be obtained.

Further, since the main component of the solid lubricant is resin, if the coating thickness is too large, welding failure will occur when assembling parts using the steel sheet after forming. FIG. 4 shows the relationship between the thickness of the solid lubricant applied and the rate of occurrence of non-energization during welding. As is clear from FIG. 4, if the coating thickness is 3 μm or less, the problem of inability to energize does not occur. Under the conditions as described above, it is possible to stably obtain good shape fixability regardless of variations in plate thickness and material properties.

[0012]

The present invention will be specifically described below. FIG. 1 is a view showing a vertical section of a composite high-strength steel sheet of the present invention. Code 1
Is a high-strength steel surface layer portion of the composite high-strength steel sheet according to the present invention, and 2 is an ordinary steel inner layer portion. Further, Table 1 shows that the composite high-strength steel sheet of the present invention is variously changed in the strength of the surface layer portion and the strength of the inner layer portion,
Further specifications of the steel sheet manufactured by changing the plate thickness ratio of the both,
A single high strength steel plate used for comparison is also shown.

FIG. 2 is a diagram showing a schematic diagram of hat type molding, and a tool as shown in this diagram was used. Reference numeral 3 is a punch for a bending tool, 4 is a blank holder, 5 is a tool die, 6 is a composite high-strength steel plate of the present invention, 7 is a clearance, and 8 is a lubricating oil or a solid lubricant. FIG. 3 is a diagram showing a hat-shaped part and a schematic diagram of the plate warp phenomenon. As shown in this figure, a 400 mm square square steel plate was press-molded into a hat type. The plate thickness at this time was 2.0 mm in all cases. The plate pressing pressure is 600 kN,
Further, the lubricant was molded under the conditions shown in Table 2. The plate warp was evaluated by the amount of plate warp of the vertical wall portion shown in FIG.

[0014]

[Table 1]

[0015]

[Table 2]

FIG. 5 is a diagram showing the relationship between the tensile strength and the amount of warpage of the test material under each manufacturing condition. That is, FIG. 5 shows the results of the amount of plate warpage when the materials of Table 1 were used and the clearance was set to 105% and the lubrication was performed under the rust preventive oil condition. The test materials 1 to 12 of the present invention have the effect of suppressing the springback due to the residual stress of the deformed surface layer portion due to the softening of the inner layer portion, so that the warpage is reduced as compared with the conventional single high strength steel sheet. However, the test materials 13-16
Since the composite high-strength steel sheet of No. 2 has a high plate thickness ratio in the surface layer portion and has a small difference from the single steel sheet, the effect of reducing warpage is small. From the above examples, it can be seen that the present invention reduces the warpage of the composite high-strength steel plate as compared with the conventional single-strength high-strength steel plate and is excellent in shape fixability.

FIG. 6 is a diagram showing the relationship between the lubricant condition and the amount of plate warpage. In FIG. 6, groups A, B, C and D of Table 1 are shown.
The results of the amount of plate warp when the clearance is 80%, the lubrication conditions are rust preventive oil and a solid lubricant, and the thickness of the solid lubricant is 3.0 μm are molded. Therefore, referring to FIG. 6, in the case of only antirust oil,
Although the clearance is smaller than the steel plate thickness, residual stress is reduced due to plastic deformation due to ironing,
Although the amount of plate warp is small, since the frictional force remains, the amount of plate warp is still large due to the compressive force from the punch side wall and the tensile force from the wall of the die. However, by using the solid lubricant, the frictional force can be greatly eased, so that the plate warpage can be greatly reduced. As described above, by adding a high lubrication condition to the composite high-strength steel sheet and the ironing process according to the present invention, it is possible to significantly reduce the warpage. still,
When the present invention is used for automobile parts, the steel plate thickness is 1.
0 to 2.6 mm is suitable.

[0018]

According to the present invention, by applying a solid lubricant to a steel sheet and setting the clearance to an appropriate dimension, it is possible to reduce the warp of the high-strength steel sheet without increasing the pressing step. . This makes it possible to apply high-strength steel plates to the inner plates and members of automobile parts, and realize weight reduction. Further, it makes it easy to apply a high-strength steel plate to a bend-formed part, and is extremely effective in expanding the applications of these.

[Brief description of drawings]

FIG. 1 is a view showing a vertical section of a composite high-strength steel sheet of the present invention,

FIG. 2 is a diagram showing a schematic diagram of hat-shaped molding,

FIG. 3 is a diagram showing a model of a hat-shaped molded part and a plate warp phenomenon;

FIG. 4 is a diagram showing a relationship between a solid lubricant thickness and a non-energization occurrence rate;

FIG. 5 is a diagram showing the relationship between the tensile strength and the amount of plate warp of the test material under each manufacturing condition,

FIG. 6 is a diagram showing a relationship between a lubricant condition and a warp amount.

[Explanation of symbols]

 1 High Strength Steel Surface Layer 2 Plain Steel Inner Layer 3 Bending Tool Punch 4 Blank Holder 5 Tool Die 6 Composite High Strength Steel Plate 7 Clearance 8 Lubricating Oil or Solid Lubricant

Claims (1)

[Claims] 1. A method of forming a composite high-strength steel sheet into a hat-shaped cross-sectional shape using a bending tool having a U-shaped cross section, wherein the steel sheet surface layer has a tensile strength from the surface layer to within 30% of the sheet thickness. 500 to 1000 MPa, the inner layer is 250
~ 400MPa composite high strength steel plate, apply solid lubricant to the punch side one or both sides to a thickness of 3μm or less, and set the clearance between the punch and the die to 80-100% of the thickness of the composite steel plate. A method for bending a composite high-strength steel sheet with excellent shape fixability.