JPH07275938A - Clad high strength steel sheet superior in shape freezing characteristic - Google Patents

Abstract

PURPOSE:To provide a clad high strength steel sheet which has a superior shape freezing characteristic and which is suitable for reducing warp in respect to bending parts in automobile components. CONSTITUTION:In a composite steel sheet to be formed into a hat shaped cross- section by the use of a bending tool having a U-shaped cross-section, the surface layer 1 of the steel sheet is made a high tension of 500 to 1,000MPa, the inner layer 2 is 250 to 400MPa and additionally the thickness of the surface layer 1 is made 30% or less of the sheet thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite high-strength steel sheet excellent in shape fixability for the purpose of preventing shape-freezing defects such as warpage when a high-strength steel sheet is used.

[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]

As described above, the use of high-strength steel sheets is limited because of problems in shape fixability. It is an object of the present invention to provide a high-strength steel sheet that has good shape fixability without providing a reworking step in bending and further restricting the use of the high-strength steel sheet.

[0005]

[Means for Solving the Problems] In a composite steel sheet formed into a hat-shaped cross-sectional shape using a bending tool having a U-shaped cross section, the steel sheet surface layer has a tensile strength from the surface layer to within 30% of the plate thickness. 500 to 1000 MPa, the inner layer portion is 25
It is a composite high-strength steel sheet excellent in shape fixability, which is characterized by 0 to 400 MPa.

[0006]

The composite steel sheet of the present invention will be described below.
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 made soft, and the tensile strength of the entire steel sheet is the same. 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 and soft inside
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 low 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. Under the above conditions, it is stable regardless of variations in plate thickness and material properties,
Good shape fixability can be obtained.

[0008]

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 schematic view of a hat-shaped molded part and a plate warp phenomenon. As shown in FIG.
It is a part that is press-molded into a hat shape by using a square steel plate of square mm. The plate thickness at this time was 2.0 mm in all cases. Molding was carried out at a plate pressing pressure of 600 kN. The plate warp was evaluated by the amount of plate warp of the vertical wall portion shown in FIG. FIG. 3 is a diagram showing the relationship between the tensile strength and the amount of plate warpage of the test material under each manufacturing condition. That is, FIG. 3 shows the results of the amount of warpage under each test condition.

[0010]

[Table 1]

In the test materials 1 to 12 of the present invention, the softening of the inner layer portion suppresses the springback due to the residual stress in the surface layer portion after the deformation, so that the warpage is reduced as compared with the conventional single high strength steel plate. ing. However, since the composite high-strength steel sheets of the test materials 13 to 16 have a high plate thickness ratio in the surface layer portion and have 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.

[0012]

According to the composite high-strength steel sheet of the present invention, plate warpage can be reduced and shape fixability can be improved without increasing the number of pressing steps and providing a reworking 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 view showing a hat-shaped molded part and a schematic diagram of a plate warp phenomenon;

FIG. 3 is a diagram showing the relationship between the tensile strength and the amount of plate warp of the test material under each manufacturing condition.

[Explanation of symbols]

 1 High strength steel surface layer 2 Normal steel inner layer

Claims (1)

[Claims] 1. A composite steel sheet formed into a hat cross-sectional shape using a bending tool having a U-shaped cross section, wherein the steel sheet surface layer has a tensile strength of 50 from the surface layer to within 30% of the sheet thickness.
0 ~ 1000MPa, the inner layer is 250 ~ 400MP
A composite high-strength steel sheet excellent in shape fixability characterized by being a.