JP2001001062A - Press forming method for tailored blanks of different materials with excellent formability - Google Patents

Abstract

(57) [Summary] [Problem] In press-forming of tailored blanks of different thicknesses and materials, tailored blanks of different thicknesses and different materials to avoid breaking on the low-strength (or low-thickness) material side due to strength-limiting. Provided is a press forming method for improving the formability of a material. SOLUTION: Base plates having different strengths (= (tensile strength) × (plate thickness)) are continuously welded and fastened, and the strength ratio is preferably 1.2.
In the method of press-molding the dissimilar-material tailored blank material described above, a welded portion is set at a position where a strain gradient is generated when press-molding with a single base plate, and a high-strength base plate is arranged on a high strain side, A low-strength raw plate is arranged on the side where distortion is low, and both raw plates are continuously welded and fastened, and then press-formed. Also,
A weld is set near the point where a strain peak occurs when press-molding with a single base plate, the base plate on the high strength side is placed on the side containing the strain peak, and the low strength side is placed on the side that does not contain the strain peak. After the base plates are arranged, the base plates are continuously welded and fastened, and then press-formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for press-forming a tailored blank material of different materials by combining two or more kinds of base plates. More specifically, the present invention relates to two or more cold-rolled steel sheets having the same or different sheet thicknesses or mechanical properties, hot-rolled steel sheets, surface-treated steel sheets, stainless steel sheets,
The present invention relates to a method for press-forming a tailored blank material obtained by continuously welding and joining aluminum sheets, aluminum alloy sheets, etc., and in particular, a sheet expressed by (tensile strength) x (sheet thickness) of the sheet The present invention relates to a press-forming method for improving the press-formability of a tailored blank material made of dissimilar materials in which two or more kinds of base plates having different strengths are fastened.

[0002]

2. Description of the Related Art In the production of pressed parts for automobiles,
In order to simplify the process and reduce the number of molds, a technique of integrally molding two or more types of parts has been widely adopted. However, when integrally molded parts are produced from blanks, the number of scraps increases, which means that thin plates of the same thickness and the same material can be welded by laser welding or mash seam to improve the yield of blanks. A method has been developed in which continuous welding is performed by welding, electron beam welding, TIG welding, arc welding, or the like, and then integrally pressed. Furthermore, recently, from the viewpoint of collision safety, tailored blanks of dissimilar materials, which are obtained by successively welding and tightening a base plate having a required material strength and a changed thickness to a part of a part, have been widely used. I have.

[0003] The tailored blanks fastened by continuous welding have economic effects such as improvement in yield, but they have poor forming at the time of press forming due to deterioration of the material of continuous welded portions (weld beads). I have a problem. One form of this molding failure is rupture during press molding, but this rupture occurs when the base material is extended in parallel with the weld bead, and the weld bead deteriorated in material leads to rupture in the "ductile rate-limiting mode". When the base plate is stretched across the weld bead portion, the base plate on the low-strength side is broken into a "stress-controlled mode" in which the base material breaks.

As one measure against such a break, for example, Japanese Patent Laid-Open No. Hei 7-26346 discloses that 2.6 ≦ f (C, S
i, Mn, P, B) ≦ 12.5 (B ≦ 0.0005%: f (C, Si, Mn, P, B)
= 100 [% C] + [% Si] +2 [% Mn] +50 [% P] +9
000 [% B], B> 0.0005%: f (C, Si, Mn, P, B) = 1
00 [% C] + [% Si] +2 [% Mn] +50 [% P] +100
An ultra-low carbon steel sheet satisfying 0 ([% B] -0.0005) +4.5) and having excellent formability after high-density energy beam welding is disclosed. However, in such an ultra-low carbon steel sheet,
Even when applied to recent high-strength blanks, it may not be possible to satisfactorily impart the necessary strength to the members, and the effect on the fracture in the "ductile limited mode" is improved due to the improved properties of the weld bead. Although it was recognized, no measures were taken against the fracture in the "stress-controlled mode", and in some cases, it was found that the base material fractured on the low-strength base plate during press forming.

[0005] In this regard, regarding the strain distribution at the time of fracture in the "stress-controlled mode", conventional knowledge (for example, Koichi Ikemoto et al., Plasticity and Working, Vol. 32, No. 370 (1991) 1383-139)
0), it is known that a distortion ratio applied to two or more types of base plates can be determined by elementary analysis based on the strength ratio of the base plates. That is, 2
The stress-strain relations for different materials are given by the suffix 1: high strength material,
Subscript 2: Assuming low-strength material, σ ₁ = K ₁ ε ₁ ⁿ¹ , σ ₂ = K ₂ ε ₂
^{It is represented by n2} .

Here, σ: tensile stress [MPa], K: plasticity coefficient [MPa], ε: plastic strain, and n: work hardening index. For the following explanation, t: plate thickness [mm], T
S: Maximum tensile strength [MPa]. Since the stress is balanced at the joint, σ ₁ t ₁ = σ ₂ t ₂ holds. Therefore, when these equations are solved, the strain (ε _1max ) in the high-strength side when the low-strength side reaches the breaking limit
Is based on the values of TS ₁ and TS _{2 and} the values of t ₁ and t ₂
Given by (1).

Ε _1max = n ₁ ｛(t ₂ / t ₁ ) (TS ₂ / TS ₁ )｝ ^{1 / n1} ... (1) However, the maximum strain (ε _1max ) in the high-strength element _plate is Even if it could be calculated, no method for improving the fracture in the "stress limited mode" was shown. Therefore, in the press work site, in the tailored blank material of different materials, if the "stress-controlled mode" fracture occurs, reduce the plate thickness ratio to lower the raw plate strength ratio or lower the strength ratio itself Or, I had to give up the application of tailored blanks.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the conventional press-formed tailored blanks made of dissimilar materials. An object of the present invention is to provide a press forming method having excellent formability, which prevents breakage of a base material that occurs in a low-strength raw plate for forming such as overhang.

[0009]

Means for Solving the Problems The present inventors have found that defective molding at the time of press molding of tailored blank materials of different materials,
In particular, the present inventors have invented a press method for improving the formability by utilizing the strain distribution generated by the shape of the press-formed product for the fracture in the "stress-controlled mode", for which effective measures could not be taken until now. That is, the gist of the present invention is as follows. (1) In the method of press-forming a tailored blank material made of dissimilar materials, which is continuously welded and fastened with different strengths (= (tensile strength) × (thickness)), distortion occurs when press-forming with a single material. A weld is set at a place where a gradient is generated, a high-strength raw plate is placed on the side with high distortion, a low-strength raw plate is placed on the low-distortion side, and both plates are continuously welded at the weld. After concluding,
A press forming method for tailored blanks of dissimilar materials excellent in formability characterized by press forming. (2) In the method of press-forming a tailored blank material of different materials, which is continuously welded and fastened with different strengths (= (tensile strength) × (thickness)), distortion occurs when press-forming with a single raw material A weld is set in the vicinity of the peak, a high-strength blank is placed on the side containing the strain peak, and a low-strength blank is placed on the side that does not contain the strain peak. A press forming method for tailored blanks of dissimilar materials having excellent formability, which is formed by continuous welding at a welded portion, and then press forming. (3) The strength ratio between the high-strength base plate and the low-strength base plate is
1.2 The press-forming method for a tailored blank material having excellent formability according to the above (1) or (2), which is not less than 1.2.

[0010] In the present invention, the blank used for the tailored blank material of different materials includes cold-rolled steel sheet, hot-rolled steel sheet, zinc / iron-zinc / aluminum / tin / zinc-nickel /
The range includes plated steel sheets such as lead-tin and stainless steel sheets, and also includes non-ferrous metals such as aluminum and thin sheets such as aluminum alloys.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. FIG. 1 shows that when a strain in a longitudinal direction of a certain press-formed product is measured, the strain of a single base plate (steel plate) shows a rising trend to the right and reaches a danger zone at l = 100 mm. The situation is shown schematically. The distortion was measured by transferring a scribed circle having a diameter of 10 mm before molding, and measuring the amount of elongation after molding with a magnifying projector.

In the case of a press-formed product having such a strain distribution, assuming a tailored blank material in which a weld bead portion is arranged at a position of l = 65 mm in FIG.
mm), a low-strength base plate (TS = 304 MPa, plate thickness = 0.8 mm) is placed, and a high-strength base plate (T
(S = 459MPa, thickness = 0.8mm), plastic strain is suppressed on the high-strength base plate, and conversely, plastic strain increases on the low-strength base plate. Distortion in the danger zone of fracture (l = 100mm) in the case of a single sheet (steel plate)
20 is suppressed to 0.14, so that breakage can be avoided.

FIG. 2 shows an example in which a strain peak is present at a position of 1 = 50 mm in the strain distribution of a single raw plate (steel plate). In this case, the position of the weld bead portion is Press the tailored blank material, which is shifted 5 mm to the left from the strain peak position, arranges a high-strength blank on the side containing the strain peak, and arranges a low-strength blank on the side not containing the strain peak. By subjecting it to forming, the strain 0.22 at the strain peak position (l = 50 mm) in a single base plate (steel plate) is reduced to 0.
13 schematically shows that it can be suppressed and breakage can be avoided.

As described above, based on the strain distribution obtained by press forming a single base plate (steel plate), the side having the higher strain or
By pressing a dissimilar tailored blank made so that a high-strength raw plate is located on the side containing the strain peak, it is possible to suppress the distortion generated in the press-formed product and ensure high formability. Becomes The (
In the invention of 2), the welded portion is set in the vicinity of the location where the strain peak occurs.
If the weld bead portion is close to the center of the peak, stress is concentrated on the weld bead portion, and there is a possibility that the weld bead portion may break due to stretch flange deformation. Therefore, it is desirable that the region be 5 mm, preferably 10 mm or more from the strain peak. Further, the effect of the present invention is weakened as the weld bead part moves away from the central part of the peak, so that the weld bead part is desirably located within 200 mm or less from the position of the strain peak.

In the invention of the above (3), the strength ratio between the high-strength base plate and the low-strength base plate is 1.2 or more. Ratio
When the ratio is 1.2 or more, the effect of the present invention becomes remarkable, so that the intensity ratio is set to 1.2. The upper limit of the strength ratio need not be particularly defined, and the effects of the present invention can be obtained. However, from the viewpoint of a practical combined strength ratio, the upper limit is preferably 10.0 or less.

In the present invention, the distortion may be measured by a direct reading method using a transparent scale for reading distortion, an automatic reading method using a phase difference with a camera or the like, in addition to the above method.

[0017]

The present invention will be described in more detail with reference to the following examples. Table 1 shows that a 0.80 mm thick soft-rolled steel sheet (A: SPCEN) used as the low-strength raw sheet and a high-strength cold-rolled steel sheet (B: TS440MPa) of the same thickness used as the high-strength raw sheet Grade, JIS standard S
AFC440R) shows the mechanical properties. A material (low strength side: SPCE
N) and B material (high-strength side: SAFC440R) were welded and joined by a 5 kW CO ₂ laser to produce tailored blanks of different materials.

[0018]

[Table 1]

FIG. 3 shows the appearance of a real press-type press-formed product (rear side member) used for evaluating the formability of the tailored blank material of the different material as viewed from above the front. FIG. 4 shows the shape of the press-formed product when viewed from the side, and the range of the center portion actually used to evaluate the formability. FIG. 5 is an enlarged view of the central portion shown in FIG.
The location of the weld bead part (part, part, and part in the figure) in the dissimilar material tailored blank material and the strain measurement direction across the weld bead part are shown.

At the center of this press-formed product,
A tailored blank material of a different material, in which a weld bead portion comes at each of the parts, the parts, and the parts, was prepared, and an actual press experiment was performed by changing the left and right material configurations. Table 2 summarizes the experimental conditions and test results of the actual press experiment.

[0021]

[Table 2]

FIGS. 6, 7 and 8 show, respectively,
Strain distribution in the above part, strain distribution in the same part,
3 shows a strain distribution in the same part. In FIG.
In the case of a single steel plate (SPCEN), since there is no strain gradient in the part, SPCEN (soft cold-rolled steel plate) and SAFC44
0R (high-strength cold-rolled steel sheet) combination (for example, in the figure, SPCEN + SAFC440R has SPCEN on the left and SAC on the right.
It means that FC440R was arranged. ) Shows that a strain peak occurs on the SPCEN side. Due to the deeper forming depth on the left side, the fracture occurred when SPCEN (soft cold rolled steel sheet) was arranged on the left side, but on the other hand, when SPCEN (same as above) was arranged on the right side, the strain peak was right. , But did not lead to breakage.

In FIG. 7, in the case of a single steel plate (SPCEN),
The part has a strain gradient that rises to the right, and according to the present invention, when SAFC440R (high-strength cold-rolled steel sheet) is arranged on the right side with high strain, the maximum strain is suppressed (see FIG. It can be seen that the molding allowance is improved. Conversely, when a low-strength SPCEN (soft cold-rolled steel sheet) is placed on the right side, the strain is concentrated on the SPCEN side and shows a high strain peak value (−,
▲-, see).

In FIG. 8, in the case of a single steel plate (SPCEN),
The portion has a strain distribution state with a strain peak. According to the present invention, the weld bead portion is set so as to be located 5 mm left of the strain peak (position 0 in the figure), and includes the strain peak. When SAFC440R (high-strength cold-rolled steel sheet) is arranged on the right side, it can be seen that forming distortion is suppressed and forming allowance is improved (see-◇-in the figure). on the other hand,
When a low-strength SPCEN (soft cold-rolled steel sheet) was placed on the side containing the strain peak on the right, the strain peak was lengthened and fractured. In addition, the fracture modes were all “stress-controlled mode” fractures in which the substrate was fractured on the low-strength base plate (in this case, SPCEN).

[0025]

As described above, the press-forming method for tailored blanks of different materials according to the present invention uses a tailored blank of different materials in which two or more kinds of base plates having different strength ratios (thickness × strength) are fastened by continuous welding. Despite the fact that it is press-formed, the strain distribution generated according to the shape of the deep-drawn part is used effectively, and it is extremely effective for forming such as deep-drawing and overhang. It is expensive.

According to the present invention, a tailored blank material made of a cold rolled steel sheet, a hot rolled steel sheet, a plated steel sheet such as zinc, a stainless steel sheet, or a thin sheet of a non-ferrous metal such as an aluminum sheet or an aluminum alloy sheet is used as a base plate. In press molding,
By optimally arranging the base plate suitable for the strain gradient of the molded product, `` stress-controlled mode '' fracture that could not be dealt with by conventional measures to improve the formability of tailored blank material
It is possible to provide a method for press-forming a tailored blank material of a different material that works effectively.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a distortion reduction effect when a dissimilar material tailored blank material having a high-strength raw plate disposed on the right side is applied to a press-formed product having a strain distribution rising upward.

FIG. 2 is a view schematically showing a strain reduction effect when a dissimilar tailored blank material in which a high-strength raw plate is arranged on a side including a strain peak is applied to a press-formed product having a strain distribution having a strain peak. It is.

FIG. 3 is a view showing the appearance of a press-formed product (rear side member) of a real press type used for evaluating the formability of the tailored blank material of different materials according to the present invention, as viewed from above the front.

FIG. 4 is a view showing a shape of the press-formed product when viewed from the side, and a range of a central portion actually used for evaluating formability.

FIG. 5 is an enlarged view of a central portion shown in FIG. 4 and shows a location of a weld bead in a tailored blank made of a different material, and a strain measurement direction across the weld bead;

6 is a diagram showing strain distribution measured after press-forming a single steel plate and a tailored blank material of a different material in which a weld bead portion is disposed at a position of a portion in FIG.

FIG. 7 is a diagram showing strain distribution measured after press-forming a single steel plate and a tailored blank material of a different material in which a weld bead portion is disposed at a position of a portion in FIG.

8 is a diagram showing strain distributions measured after press-forming a single steel plate and a tailored blank material of a different material in which a weld bead portion is arranged at the position of the portion in FIG.

Continued on the front page (72) Inventor Tohru Yoshida 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Yasunobu Miyazaki 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation F-term in the Technology Development Division (reference) 4E001 AA03 BB07 CA01 CA02 CC04 4E068 BE00 DA00 DB05

Claims (3)

[Claims] Claims: 1. A method for press-forming a tailored blank material of different materials, which is made by continuously welding and fastening steel plates having different strengths (= (tensile strength) × (plate thickness)). Welding is set at a place where a strain gradient occurs, a high-strength raw plate is placed on the high strain side, a low-strength raw plate is placed on the low strain side, and both raw plates are continuously connected at the welded portion. A press forming method for tailored blanks of dissimilar materials having excellent formability, which is performed by press forming after welding and fastening. 2. A method of press-forming a tailored blank material of different materials, which is obtained by continuously welding and fastening base plates having different strengths (= (tensile strength) × (plate thickness)). A weld is set in the vicinity of the point where a strain peak occurs, a high-strength blank is placed on the side containing the strain peak, and a low-strength blank is placed on the side that does not contain the strain peak. A press forming method for a tailored blank material having excellent formability, characterized in that press forming is performed after continuous welding and fastening at the welded portion. 3. The press of a tailored blank material with excellent formability according to claim 1, wherein the strength ratio between the high-strength raw plate and the low-strength raw plate is 1.2 or more. Molding method.