TW201330945A - Metal sheet for press forming - Google Patents

Abstract

The present invention predicts a curving property required of a metal sheet to prevent a crack of influencing curving property during press forming the metal sheet and also provided a metal sheet without a crack of influencing curving property. A metal sheet for press forming is provided. A prediction formula is derived. The prediction formula is that R/t ≤ (- &egr; f+2 (1- &egr; f) R0/t) /2 (1+ &egr; f+2 &egr; fR0/t). By using a minimum radius of curvature R0 of a press forming mold and a limit strain &egr; f of a strain area of a plane surface in a forming limit diagram, the prediction formula calculates a strain of a surface of the metal sheet during a crack appears wherein the crack influences expandability of the metal sheet. Thus a crack that influences curved ability is judged not to appear when the strain of the surface of the metal sheet is under a limit surface strain &egr; critical. The metal sheet that satisfies the prediction formula is selected as the sheet for press forming according to a maximum curved ability R/t which is required to restrain the crack that influences the curved ability.

Description

Press forming metal sheet

The present invention relates to a metal sheet for press forming which can be press-formed into a desired shape without causing cracks on a metal plate.

Press forming is one of the representative metal working methods in which a metal plate is sandwiched between a pair of dies, and a metal plate such as a steel plate is formed in conformity with the shape of the mold to obtain a desired shape. Press forming is used in a wide range of manufacturing fields such as automotive parts, machine parts, building components, and home appliances.

In recent years, particularly in press forming of automobile parts, the utilization range of high strength steel sheets has been expanded, but there has been a problem that the press formability is lowered as the strength of the material to be processed is increased. The countermeasures include a steel sheet (a dual phase steel sheet) composed of a hard phase and a soft phase, or a steel sheet with a retained austenite (transformation induced). Plasticity, TRIP)), etc., which simultaneously achieve strength and elongation, and improve the mechanical properties of the metal sheet itself. This is because the press formability is related to the elongation of the metal sheet.

Generally, the press formability of a metal plate is represented by a forming limit diagram. The forming limit line diagram is a line drawing in which various biaxial stresses are applied to the metal plate, and a crack is generated in the metal plate or a strain before the crack is generated as a limit value. Has been actively working to improve this An attempt to measure or predict the accuracy of the forming limit, and to verify its influence with various material properties (for example, refer to Non-Patent Document 1).

In addition, a press forming simulation using a finite element method (Finite Element Method) is carried out by using the forming limit line diagram, and molding conditions in which no crack is generated on the metal plate are sought (for example, see Patent Document 1).

Prior art literature Non-patent literature

Non-Patent Document 1: "Iron and Steel", Li Wei Gu, 3 others, Japan Iron and Steel Association, Vol. 88 (2002), No. 2, p88~94, "Forming Limits for Press Forming of Sheet Materials "Prediction of forming Limit Diagram (FLD) and Effect of Work Hardening Property on FLD of Sheet Metals"

Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-152407

The inventors and the like performed press forming of a high-strength steel sheet by various forming conditions, and as a result, it was found that there were a large number of cases which largely deviated from the prediction results of cracks simulated using press forming, and produced as shown in Fig. 1(a). The ductility governs the cracks of different cracks. Repeatedly conducted research, it is clear that the crack has a strong correlation with the bending property of the metal plate, which is caused by the cracking of the crack from the surface of the plate as shown in Fig. 1(b). The crack is also caused by the press forming of a high-strength steel sheet excellent in elongation. In other words, in the conventional method of performing the evaluation of press formability or the prediction of cracks based on the ductility of the metal sheet, it is possible to form the press sheet even if the metal sheet has excellent ductility. Cracks are generated midway.

The present invention has been made to solve the above problems, and an object of the present invention is to predict a bending property required for a metal plate in order to prevent occurrence of cracks under bending, and to provide a metal plate which does not cause cracking in the middle of press forming.

In order to solve the above problems, the inventors have conducted various studies on the occurrence of cracks governed by bending, and as a result, have found that the correlation with the bending property R/t of the metal plate is strong. Here, the bendability R/t is the mechanical characteristic of the metal sheet obtained by the bending test, and the minimum bending radius at which no crack occurs on the surface of the metal plate (the minimum bending radius (the ultimate bending radius which is bent without cracking) )) R is divided by the plate thickness t. The inventors and the like performed a 90-degree V bending test on various metal plates and found that if the strain on the outer side surface of the curved surface when the surface of the metal plate is cracked is defined as the ultimate surface strain ε _critical , the strain on the surface of the metal plate at the time of press forming When the ultimate surface strain ε _critical is exceeded, a crack dominated by bending is generated.

Therefore, the following prediction formula is invented: the strain of the surface of the metal sheet at the time of press forming is determined according to the minimum radius of curvature R _{0 of the} die for press forming and the ultimate strain ε _f of the plane strain region in the forming limit diagram. When the strain on the surface of the plate is _{equal to} or less than the ultimate surface strain ε _critical , it is possible to determine that cracks do not cause bending.

R/t≦(-ε _f +2(1-ε _f )R ₀ /t)/2(1+ε _f +2ε _f R ₀ /t)

In order to solve the above problems, the present invention predicts the maximum bendability R/t required for the metal sheet in order to prevent the occurrence of cracks under bending, and selects a metal sheet having flexibility satisfying the above formula as a press forming. Use a metal plate.

According to the present invention, when the metal sheet to be subjected to the press forming is formed, the occurrence of cracks governing the bending property can be prevented, so that the press forming can be stably performed, and the reduction in the defective rate of the press-formed product can be greatly contributed.

In addition, the bendability R/t required for the metal sheet can be accurately predicted at the material design stage, and contributes to shortening the development time of the metal sheet.

In addition, it is possible to accurately predict whether or not the selection of the metal plate used for press forming various parts such as panel parts and structural-frame parts of automobiles is appropriate.

Hereinafter, embodiments of the present invention will be described based on the drawings.

(Method of Making Forming Limit Line Diagram)

In the production of the forming limit diagram (FLD), first, the metal sheets are processed into test pieces having various widths as shown in Fig. 2 and having a width of 10 mm to 100 mm. Here, the reason for preparing various test pieces of varying width is to make the strain ratio (minimum main strain and maximum) The ratio of the main strains varies within a wide range.

Next, the surface of the metal plate is marked. The shape of the mark is a circular pattern, a dot pattern, a grid pattern, a concentric pattern, or the like as long as the strain can be measured after forming. Further, the marking method may be electrolytic etching, photo etching, ink transfer (stamp printing), or the like, and any method may be used, but the cracking may cause cracking, which is not preferable.

Next, the test piece is stretch-formed using a ball-shaped punch having a radius of curvature of 25 mm or more at the tip end, and the time at which the plate is cracked, or the groove is generated, or the surface of the plate is cracked. , the end of the formation. Further, the reason why the minimum radius of curvature of the tip end of the punch is limited to 25 mm is that if it is less than 25 mm, the influence of the bending deformation in the deformed region of the tip end portion of the punch cannot be ignored.

After the completion of the above-described stretch forming, the mark position or shape change of the portion where the tip end of the punch abuts is measured, and the maximum principal strain and the minimum principal strain are obtained. By repeating the above operations for test pieces of various widths, the maximum principal strain and the minimum principal strain can be obtained in a wide range. Next, the measurement results of the maximum principal strain and the minimum principal strain obtained in the above manner are expressed in two dimensions, and a forming limit diagram as shown in FIG. 3 is obtained.

Here, a region where the minimum principal strain such as bending deformation is close to 0 is referred to as a plane strain region, and the ultimate strain thereof is represented by εf of FIG. In the method of predicting a conventional crack in which the ductility of the metal sheet is used as a criterion, the forming limit line of the forming limit diagram is sandwiched, and it is confirmed whether or not it exists in any of the crack generating region and the crack-free region. On the side, when it exists in the crack generation region, cracks are predicted to occur.

(predictive export)

An embodiment for predicting the required bendability of a metal plate will be described.

When the limit bending radius in the 90-degree V bending test of the target metal plate is R and the thickness of the metal plate is t, the ultimate surface strain ε _{critical of the} curved outer side is _expressed by the formula according to the pure bending theory. ) said.

ε _critical =t/(t+2R).........(1)

Further, regarding the metal plate subjected to bending deformation in the plane strain region at the time of press forming, if the minimum bending radius (minimum radius of curvature) of the mold is set to R ₀ , the ultimate strain ε _{R0 of the} surface of the metal plate is expressed by the formula (2) This formula (2) is formed by adding the strain portion of the bending strain to the ultimate strain ε _f of the plane strain region in the forming limit diagram.

ε _R0 = ε _f + t / (t + 2R ₀ )... (2)

Here, ε _critical is a limit value of the strain of the crack which does not cause the bending property in the plane strain region, and therefore the condition for not causing the crack to affect the crack becomes the formula (3).

ε _critical ≧ε _R0 .........(3)

According to the formula (1) to formula (3), in order to prevent cracks dominated by bending The bendability R/t required for the metal plate is obtained by the following formula (4).

R/t≦(-ε _f +2(1−ε _f )R ₀ /t)/2(1+ε _f +2ε _f R ₀ /t)...(4)

Therefore, by setting the bendability of the metal plate to a range satisfying the bendability R/t represented by the formula (4), a metal plate which does not cause cracking at the time of press forming can be obtained.

Further, the mold to which the present embodiment is applied is assumed to be a mold having a minimum bending radius (minimum curvature radius) R _{0 of} less than 25 mm. The reason for this is that when a metal plate is press-formed using a mold having a minimum bending radius (minimum radius of curvature) R ₀ of 25 mm or more, the influence of bending deformation is small, and it is likely to be a crack shape governed by ductility.

In addition, the metal plate to which the present embodiment is applied is a metal plate having a thickness t of 0.5 mm or more, a tensile strength of 980 MPa or more, and an ultimate bending radius R of 1 mm or more. The reason for this is that if the thickness t is less than 0.5 mm, even if bending deformation is applied to the metal plate, the strain generated on the outer surface of the curved portion is small, and cracks governing the bending property are hard to occur. In addition, a raw material such as a tensile strength of less than 980 MPa is generally excellent in flexibility, and in a raw material having excellent bending property R/t such as an extreme bending radius R of less than 1 mm, it is usually used by press forming. Within the range of the minimum bending radius (minimum radius of curvature) of the mold, the bending-dominated crack does not become a problem.

(Example)

Three kinds of test materials A, test materials B, and test materials C shown in Table 1 which were manufactured in a manner different in flexibility were used. First, in the shape shown in Fig. 2, a plurality of test pieces having a width of 10 mm to 100 mm in the narrowest portion were produced, and a dot pattern was marked on the surface of the test piece by electrolytic etching at a distance of 1.0 mm between the punctuation marks. Next, the test piece was stretch-formed using a ball-end punch having a minimum curvature radius of 25 mm at the front end. Further, in the stretch forming using the ball-end punch, the steel sheet is molded until a through crack occurs. Next, the test piece after the stretch forming was subjected to measurement of the change in the dot interval in the vicinity of the tip end of the punch, and the maximum principal strain and the minimum principal strain were obtained, and a forming limit diagram was prepared.

The forming limit diagram of the test material A produced by the above method is shown in Fig. 4(a), and the forming limit diagram of the test material B is shown in Fig. 4(b), and the test material C is formed. The limit line diagram is shown in Figure 4(c). Further, the plane strain region is theoretically a region having a minimum principal strain of 0, but actually is affected by friction or the like, and does not necessarily coincide with an axis having a minimum principal strain of 0. Here, when measuring the ultimate bending radius R and the bending property R/t of the test material, it is not limited to the 90 degree V bending test, and may be a bending test method such as a U bending test. Table 2 shows the ultimate strain ε _f in the plane strain region based on the forming limit line graph of each test material.

Next, using the ultimate strain ε _f , the thickness t, and the minimum bending radius (minimum radius of curvature) R _{0 of the} mold, the required bending property for producing a desired product by press forming is predicted according to the formula (4). When press forming with a mold having a minimum bending radius (minimum radius of curvature) R ₀ of 2 mm, 3 mm, 4 mm, ..., 21 mm, the bending property required for the metal sheet can be predicted as shown in Table 3 R/ The value on the right side of equation (4) of t. The lower line of Table 3 is a result of the prediction that the bending property R/t of the test material is insufficient. According to the results, when the minimum bending radius (minimum curvature radius) R ₀ of the mold is 4 mm or more, the test material A does not cause cracks governed by the bending property. In addition, when the minimum bending radius (minimum radius of curvature) R ₀ of the mold is 7 mm or more, the test material B does not have cracks governed by bending, and the minimum bending radius (minimum radius of curvature) R ₀ of the mold is 20 mm. In the above case, the test material C did not cause cracks governed by bending.

(Verification of the embodiment)

The prediction result of the bending property R/t required for the above-mentioned steel sheet was verified using the mold shown in Fig. 5 in which the flat workpiece material was molded into a hat mold shape. After the test material was processed into a rectangular shape as shown in Fig. 6, the test material was formed using a punch having a punch shoulder radius, that is, a minimum bending radius (minimum radius of curvature) of the mold, R ₀ of 5 mm and 10 mm. For the lubrication, a normal rust preventive oil was used, and the wrinkle pressing load was set to 15 tons, and the forming height was set to 50 mm, and it was confirmed whether or not the test material was subjected to cracking.

The results of the experiment are shown in Table 4, and the case where the crack which is subjected to the bending property is set is ×, and the case where the crack which does not cause the bending property is formed can be formed as ○. When the minimum bending radius (minimum radius of curvature) R ₀ of the mold was 5 mm, the test material A was formed without cracks, but the test material B and the test material C were cracked from the surface of the metal plate. Similarly, when the minimum bending radius (minimum radius of curvature) R ₀ of the mold was 10 mm, the test material A and the test material B were formed without cracks, but the test material C was cracked from the surface of the metal plate.

A schematic view of the vicinity of the shoulder of the punch is shown in Fig. 7 for the test material until the crack is caused by the bending. Each of the test materials was cracked from the surface and cracked by bending. According to the result It is understood that by using the prediction formula (4) of the invention, the bending property R/t required to prevent cracking which is dominant in bending property is predicted, and a metal plate capable of preventing cracks which are subject to bending property can be selected.

(industrial availability)

The present invention is not limited to the above description, and for example, an example in which the steel sheet (steel plate of 1180 MPa grade) having a tensile strength of 980 MPa or more is used in the above embodiment, and the present invention is preferably applied to such a high. Press forming of strength steel sheets, but it can also be applied to metal sheets other than steel sheets.

Figs. 1(a) and 1(b) are views showing a crack form of a ductile metal plate and a crack form of a metal plate which is controlled by bending.

Fig. 2 is a view showing the shape of a test piece used in the production of a forming limit diagram.

Fig. 3 is a view for explaining a forming limit line diagram.

4(a) to 4(c) are views showing a forming limit diagram of the test material A, the test material B, and the test material C.

Fig. 5 is a view showing the shape of a hat molding die.

Fig. 6 is a view showing the shape of a test piece for cap forming.

Fig. 7 is a schematic view showing a test material formed into a shape of a cap mold until cracks are generated.

Claims (1)

A metal sheet for press forming, characterized in that a minimum bending radius at which the surface of the metal sheet for press forming is not cracked is defined as R, and a plane obtained by forming a forming limit diagram of the metal sheet for press forming is prepared. The ultimate strain in the strain region is defined as ε _f , the minimum radius of curvature of the press-molding mold is defined as R ₀ , the thickness of the press-forming metal sheet is defined as t, and the bending property of the press-formed metal sheet is R/ t is set to satisfy the range of the following formula: R / t ≦ (- ε _f + 2 (1 - ε _f ) R ₀ / t) / 2 (1 + ε _f + 2 ε _f R ₀ / t).