JP2000263134A - Manufacture of press formed body of steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manfacturing method for reducing the camber of a sheet and controlling it in a small range without increasing the process of press. SOLUTION: At the time of press-forming the steel sheet, in this manufacturing method of a press formed body of the steel sheet, the strength in the deformed part of the formed body by bending and bending back is lowered. By heating the sheet temperature to 300-900 deg.C, the strength is lowered. Taking the sheet temperature as 300-900 deg.C is to the time of the maximum in the tensile deformation from the starting time of the bending deformation after completing deformation by bending back.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet, preferably a high-strength steel sheet pressed body (for example, a hat-bent formed body).

[0002]

2. Description of the Related Art In recent years, efforts have been made to increase the strength of steel sheets in order to improve collision safety, reduce weight, and protect the environment of automobiles. For this reason, high-strength steel sheets are increasingly used for automobile inner plates, automobile parts, and strength members, but improvements in shape freezing defects are being demanded. Due to poor shape freezing, it is extremely difficult to produce a press-formed body of a high-strength steel sheet having a tensile strength of 590 MPa or more. In particular, when a component having a hat-shaped cross section such as a member is manufactured by hat bending as shown in FIG. 1, a shape freezing defect called plate warping and spring back as shown in FIG. 2 is likely to occur. Of these, the springback is formed at an angle smaller than the angle required for the molded product, and the mold is designed to have the desired angle after the external force is unloaded. Can be. On the other hand, plate warpage is a phenomenon in which a vertical wall portion of a molded product warps due to a distribution of stress due to bending / bending-back deformation in the thickness direction, and cannot be solved by a prospective technique like springback. In order to solve this problem, Japanese Patent Laid-Open Publication No. Sho 56-117831 proposes a method in which the bending radius at the die shoulder and the clearance between the punch and the die are set within a specific range to prevent warpage. However, in this case, since the shoulder radius of the die needs to be reduced, problems such as mold wear and mold galling occur when a high-strength steel plate is continuously press-formed. Therefore, a method is usually used in which a separate restriking process is provided and a shape is corrected by applying a bending deformation to the side opposite to the plate warpage. However, this method has significant drawbacks in equipment cost and productivity due to the additional steps.

[0003]

SUMMARY OF THE INVENTION The present invention relates to a steel sheet suitable for reducing sheet warpage and controlling it to a small range without increasing the number of press steps in the production of a pressed steel sheet (for example, a hat bend). The present invention relates to a method for manufacturing a press-formed body. In particular, there is a need for a method of manufacturing a press-formed body that makes it easy to apply a high-strength steel sheet to automobiles and the like, and that allows the use of high-strength thin steel sheets to be expanded.

[0004]

According to the present invention, in order to solve the above-mentioned problems, 1) a steel plate characterized in that, when press-forming a steel plate, the plate strength of a bent / bent-back deformed portion of the formed body is reduced. A method for producing a press-formed body, 2) a method for producing a pressed steel sheet according to the above 1), wherein the sheet temperature is reduced to 300 ° C to 900 ° C to reduce the sheet strength; 3) a sheet temperature of 300 ° C to 900 ° C. The method for producing a steel sheet press-formed body according to the above 2), wherein the step of maintaining the temperature in ° C. is from the start of the bending deformation to the maximum tension in the tensile deformation after the completion of the bending-back deformation.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. First, a description will be given of a plate warpage generation mechanism. As shown in FIG. 3, the material of the wall portion where the board warp is a problem undergoes bending deformation at the die shoulder, and is bent back when passing through the die shoulder, and straightened.
Immediately thereafter, it undergoes tensile deformation. This series of deformation produces a stress distribution in the thickness direction. The higher the strength of the steel sheet, the greater the difference in stress between the front and back of the sheet, so that the sheet warpage increases. From the above-described mechanism of occurrence of sheet warpage, it has been found that the sheet warpage becomes smaller when room temperature press-forming is performed at a temperature at which the strength of the steel sheet becomes lower. As a result of further study, in order to reduce the distribution of stress in the thickness direction, the higher the temperature of the material, the better. However, if the temperature exceeds 900 ° C, the oxide film tends to be thick and dense, so that the coating adhesion may decrease. In some cases, an acid pickling film removing step is required before coating, which is not preferable. Therefore, the upper limit is 900
° C. The lower limit is set to 300 ° C. or higher where a sufficient effect can be expected from the temperature dependence of the strength of the thin steel sheet. further,
If the entire material is maintained at a high temperature and processed, the sheet warpage can be reduced, but heating the entire material is industrially disadvantageous from the viewpoint of the durability and economy of the mold. Therefore, various parts of the material are controlled at various temperatures, and experiments and analyzes are performed, and the sheet temperature at which the sheet warpage can be reduced is set to 300 ° C. or higher.
We found the minimum place and time to be 900 ° C. That is, if the temperature is maintained until the point at which the tension is maximized in the tensile deformation after the material is subjected to the bending and bending back deformation, even if the temperature decreases during molding, the thickness direction of the stress inside the material does not change even if the temperature decreases. It has been found. Therefore, in order to reduce the warpage, if only the portion B shown in FIG. 4 is maintained at a predetermined temperature, it is not always necessary to control the temperatures of the portions A and C. Further, it is preferable not to heat the portion A because the friction characteristics of the lubricant are not deteriorated without heating.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below by way of specific examples. A high-strength steel sheet with a thickness of 1.4 mm, a yield strength (YS) of 509 MPa and a tensile strength (TS) of 598 MPa is cut into 500 mm squares to form a material for molding. A tool as shown in FIG. Hat bending was performed using a punch shoulder radius and a die shoulder radius of 3 mm and a molding height of 100 mm. The dies a, b, c and the punch d shown in FIG. 5 were heated and cooled to partially control the temperature of the material. Under all conditions, the wrinkle holding force (BHF) was adjusted so that the tensile stress applied to the wall material was 200 MPa. As a result, a tensile stress of 200 MPa is applied to the wall material immediately after passing through the die shoulder, and thereafter the tensile stress becomes constant. In order to evaluate the degree of warpage, the curvature of the wall of the molded product was measured. Table 1 shows each test condition, and FIG. 6 shows the curvature of the wall under each test condition. Test condition No. At a temperature lower than 300 ° C., such as 1, 2, and 3, the board warpage is large. Test condition No. 1 in which the temperature of the entire material was maintained at 300 ° C. or higher. 4,5,
Test conditions No. 6, 7, 8 and the material of the die shoulder only were maintained at 300 ° C. or higher. In 9,10,11,12,
If the sheet warpage is small and the temperature is the same, if only the material of the die shoulder is kept at the temperature, the effect of reducing the sheet warpage is sufficiently exhibited. With respect to the coating adhesion, after performing the above-described hat bending using a tool as shown in FIG. 1, a chemical conversion treatment with zinc phosphate and a cationic electrodeposition coating of 20 μm were applied. Then 4
It was immersed in warm water of 0 ° C. for 500 hours, and the wall of the molded body (FIG. 4)
(C section) was evaluated by cross-cut peeling, and a peeling area ratio of less than 10% was evaluated as ○, and a peeling area ratio of 10% or more was evaluated as x.

[0007]

[Table 1]

[0008]

According to the present invention, the warpage of a high-strength steel sheet can be reduced without increasing the number of pressing steps. This makes it possible to apply a high-strength steel plate to automobile parts and the like, and to improve the collision safety and reduce the weight of automobiles. In addition, it makes it easy to apply high-strength steel sheets to automobile parts, and is extremely effective in expanding the applications of high-strength steel sheets.

[Brief description of the drawings]

FIG. 1 is a schematic view of hat bending.

FIG. 2 is a view showing a shape freezing defect of a hat bent molded product.

FIG. 3 is a diagram showing a deformed state of hat bending.

FIG. 4 is a diagram showing a heating location of a material.

FIG. 5 is a diagram showing temperature control.

FIG. 6 is a diagram showing a warpage amount under each test condition.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Yuichi Taniguchi 5-3 Tokaicho, Tokai City, Aichi Prefecture Inside Nippon Steel Corporation Nagoya Works (72) Inventor Hirofumi Kawaguchi 1 Toyota Town, Toyota City, Toyota City, Aichi Prefecture Toyota (72) Inventor Shinji Shinbata 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 4E063 AA01 BA01 CA06 CA20 JA03 KA03 MA18

Claims (3)

[Claims] 1. A method for producing a steel sheet press-formed body, which comprises, when press-forming a steel sheet, reducing the sheet strength of a bent / bent-back deformed portion of the formed body. 2. The method for producing a pressed steel sheet according to claim 1, wherein the sheet strength is reduced by setting the sheet temperature to 300 ° C. to 900 ° C. 3. The method according to claim 2, wherein the plate temperature is maintained at 300 ° C. to 900 ° C. from the start of the bending deformation to the maximum tension in the tensile deformation after the completion of the bending-back deformation. A method for producing a pressed steel sheet.