JP2000249636A - Method for predicting and evaluating dentability and method for selecting metal plate for press forming - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently select an optimum material and its plate thickness by considering strength characteristics considered for work hardening or bake hardening of the material by calculating a dentability evaluation index and using a value of the index. SOLUTION: A metal plate having a relation between a corresponding plastic stress and a corresponding plastic strain represented by σeq=f(εeq) with an initial plate thickness t0, wherein σeq is the stress and εeq is the strain and a cured amount by heat treating for coating baking represented by σeq=g(εeq) is used, a strain introduced in the case of press forming the plate is measured, and the strain after press forming and the plate thickness t are obtained from the measured strain. The stress σeq after forming and a curing amount σHB when the formed article is coating baked are obtained by using the σeq=f(εeq) and σHB=g(εeq) from the strain εeq. Further, characteristic against dent evaluation index X defined by S=h(σeq,σHB.t) is calculated, and the characteristic against dent predicted evaluation of the press formed article is executed by using the index.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a metal plate used to satisfy dent resistance when a panel constituting an outer surface of an automobile, a home electric appliance or the like is obtained through a process of pressing a metal plate and baking paint. The present invention relates to a method for selecting the initial thickness.

[0002]

2. Description of the Related Art Dent is a phenomenon in which a local force is applied to a panel, and a dent remains when the panel is unloaded.
In order to avoid this phenomenon, it is necessary to use a metal plate having appropriate thickness and strength characteristics. In particular, the strength properties include not only yield strength and tensile strength, but also the material undergoes press forming and paint baking in the process of manufacturing this panel, so it hardens due to plastic strain received in press forming (work hardening) and paint baking. It is also necessary to consider the occasional hardening (baking hardening) by heat treatment at 170 ° C. for about 20 minutes.

[0003] The importance of such properties is disclosed in Japanese Patent Application Laid-Open No. 8-41585, such as a steel sheet having excellent work hardening properties,
It is also clear from the fact that a steel sheet having bake hardening characteristics is proposed as a steel sheet having good dent resistance in JP-A-10-46289. For example, when it is desired to reduce the weight of a product, a press product manufacturer or a material manufacturer can use only press formability and yield strength to minimize the thickness of the material used while satisfying the prescribed dent resistance. Therefore, it is important to select a material in consideration of work hardening and bake hardening.

In order to select such materials, it is necessary to press-mold and bake harden materials having various thickness and strength characteristics to actually produce panels, and to measure the dent behavior of the panels. As a method for measuring the dent amount, Japanese Patent Application Laid-Open No. 7-225181 discloses a method and an apparatus for measuring the dent property of a metal material. A method is conceivable in which a material that satisfies predetermined reference conditions is selected based on the measured values of the dent amounts of various materials obtained in this manner.

[0005]

In recent years, many materials having excellent dent resistance have been developed. However, in selecting an optimum material from materials having various strength characteristics and plate thicknesses, only such an experimental method is used. If it is intended to use, it is necessary to perform not only press molding but also baking hardening treatment and dent amount measurement experiment for many materials, and the operation is extremely complicated. In addition, once the dent amount measurement experiment is performed, indentations remain on the panel, and it is difficult to use the panel for other purposes. Therefore, we want to minimize the number of experiments other than final performance confirmation as much as possible.

It is not impossible to carry out computer simulation of the molding process and the dent experiment process by using the finite element method to reduce the experiment work required for material selection work. However, even when using computer simulation, it is complicated to create material data reflecting the bake hardening process and input data of boundary conditions and initial conditions from the results of the simulation of the press process, and to simulate the dent test process. is there. An object of the present invention is to solve the above-described problems of the prior art, and to efficiently select an optimal material and its thickness in consideration of strength characteristics in consideration of work hardening and baking hardening of the material. With the goal.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive studies and as a result, if the strain of a press-formed product can be known in advance, the work hardening of the material by a uniaxial tensile test is performed. The present inventors have found that an index having a high correlation with the dent resistance can be obtained by simple calculation based on the data of baking hardening and baking hardening, and the present invention has been accomplished. That is, the present invention provides a method for predicting and evaluating dent resistance of a press-formed product and a method for selecting a metal plate for press-forming, which are characterized by the following points.

(1) The initial plate thickness is t ₀ , and the relationship between the equivalent plastic stress σ _eq and the equivalent plastic strain ε _eq is σ _eq = f (ε _eq )
And the amount of curing by heat treatment for baking paint is σ
Using a metal plate represented by _HB = g (ε _eq ), the strain introduced when the metal plate was subjected to press forming was measured, and from the measured strain, the equivalent plastic strain ε _eq after press forming and the plate The thickness t is determined, and σ _eq = f from the equivalent plastic strain ε _eq
Using (ε _eq ) and σ _HB = g (ε _eq ), the equivalent plastic stress σ _eq after molding and the hardening amount σ _HB when the molded product is subjected to paint baking are determined, and then S = h (σ _eq , σ _HB · t)
A method for predicting and evaluating dent resistance of a press-formed product, wherein a dent resistance evaluation index S defined by the following formula is calculated, and the value of the dent resistance evaluation index S is used.

(2) A method for predicting and evaluating the dent resistance of a press-formed product, wherein a strain calculated by a forming simulation is used instead of the actually measured value of the strain introduced by the press forming in the above (1). (3) (1) or the plate surface of the maximum principal strain epsilon ₁ and the minimum principal strain epsilon ₂ and r value in calculating the equivalent plastic strain values from the strain were measured or calculated strain in (2) Average the value r expression with _{m ε eq = (r m +1} ) √1
_{/ (2r m +1) (ε} 1 2 + 2r m / (r m +1) ε 1
ε ^₂ + ε _{2 2)} dent resistance predictive evaluation methods of the press-molded product, which comprises using a.

(4) In the above (1) to (3), σ _eq =
A method for predicting and evaluating the dent resistance of a press-formed product, wherein f (ε _eq ) is an equation σ _eq = K (ε ₀ + ε _eq ) ⁿ using K, ε ₀ and n as material parameters. (5) In the above (1) to (4), assuming that σ _BH = g (ε _eq ), if ε _eq <0.01, σ _BH = 0 and ε _eq ≧ 0.01
Then, a method for predicting and evaluating the dent resistance of a press-formed product, characterized by using the equation σ _BH = c using the baking hardening amount c of the metal plate after 2% prestrain as a material parameter.

(6) In the above (1) to (5), the anti-density
S = h (σ) defining the property evaluation index S _eq, Σ_BH, T) and
The equation S = √ using m satisfying 1.0 ≦ m ≦ 2.4
(R_m+1) / 2 (σ_eq+ Σ_BH) T^mIt is special to use
Predictive evaluation method for dent resistance of press-formed products. (7) Determined using the method described in (1) to (6) above.
The dent resistance prediction evaluation index S is a predetermined reference value S
^*S ≧ S^*The type of metal plate and the first to satisfy
Initial plate thickness t₀Press forming metal characterized by selecting
This is a method of selecting a genus plate.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described with reference to the attached drawings. Referring to FIG. 1, first, one candidate of a combination of a material to be applied and a thickness thereof is appropriately selected (Step S10). At this time, candidates may be selected in consideration of other important items such as press formability and cost. Next, a mechanical test is performed using this material. Step S2
0], the average r value r _m (step S22) in the plate surface, the relationship equation between the equivalent plastic stress sigma _eq equivalent plastic Fuzumi ε _{eq σ eq} = f
(Ε _eq ) Step S24], Relational expression σ _BH between bake hardening amount σ _BH and equivalent plastic strain ε _eq = (ε _eq ) Step S2
6]. In step S20, r _m ,
Data for obtaining σ _eq = f (ε _eq ) and σ _BH = g (ε _eq ) are collected. As a test method, a uniaxial tensile test is simple and preferable.

Specifically, the r value is 0 ° from the rolling direction,
R value r in the direction of 45 ° and 90 °_0,r _45,r₉₀And σ_eq=
f (ε_eq) To determine the data for a particular direction
The stress-strain curve obtained by the tensile test is represented by σ_BH= G (ε_eq)
2% tensile prestrain as data to determine
When applied and baked and hardened at 170 ° C for 20 minutes
It is preferable to measure the amount of bake hardening in a tensile test.
Not surprisingly, these data can be obtained by means other than uniaxial tension.
Or baking at 170 ° C for 20 minutes
Thermal wear based on actual paint baking instead of curing
Bake hardening amount by performing another heat treatment simulating the history
May be measured. Naturally, step S
R of candidate material at 10_m, Σ_eq= F (ε_eq), Σ_BH=
g (ε_eq) Is known or can be estimated,
Test step S20] to obtain these again
There is no.

The parameter r _{m of the} plastic anisotropy obtained in step S22 is obtained by r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4. This value will be used in the analysis of the deformation behavior of the material in a later step. Here, the deformation behavior of the material is the equivalent plastic stress σ _eq using the in-plane isotropic quadratic yield function and the equivalent plastic strain σ _eq based on it. It is based on the fact that it can be described simply and with high precision by _eq . The use of more complex anisotropy of expression, there is also a possibility that the prediction evaluation accuracy further improves, wherein the sufficiently high accuracy as a representative only r _m assuming the plate plane isotropy If, because the convenience is impaired in terms of other more complex anisotropy, using r _m.

In step S24, stress σ _eq = f (ε _eq ) is assumed assuming a function formula including appropriate material parameters.
Fit the strain curve to determine the material parameters to get the complete σ _eq = f (ε _eq ). The stress-strain relationship may not be represented by a mathematical expression, but may be directly read from the curve using a stress-strain curve plotted on a graph as needed. It is simpler to represent. Equivalent plastic stress sigma _eq and the equivalent plastic strain epsilon _eq, assuming the plate plane isotropy, wherein using the plate plane average value r _m of r value ^{_{ε eq = √σ 1 2 -2r m}} / ( r _{m
+1) σ 1 σ 2 + σ} 2 2 and _{ε eq = (r m +1)} √1
_{/ (2r m +1) (ε} 1 2 + 2r m / (r m +1) ε 1
may obtained by ε ^₂ + ε _{2 2).} This is because, in the case of a uniaxial tensile test, σ _eq = σ ₁ and σ _eq = ε ₁ , thereby facilitating the processing.

Using a more sophisticated anisotropic yield function,
This plastic stress ε_eqAnd equivalent plastic strain ε _eqMay be defined
However, there are many parameters.
Need to be taken into account.
not enough. In practice, the above formula is sufficient. Work hardening
Function f (ε_eq) As e.g._eqHigher-order polynomials and their
Other formats may be used, but the approximation accuracy is high and
Swift equation σ often used in simulation_eq=
K (ε₀+ Ε_eq)ⁿIt is preferable to use

In step S26, a function formula including appropriate material parameters is assumed as σ _BH = g (ε _eq ), and the material parameters are determined by fitting to a curve showing the relationship between the pre-strain and the bake hardening amount. Σ _BH = g (ε _eq )
Get. The relationship between the prestrain and the bake hardening amount may not be expressed by a mathematical expression, but may be directly read from the curve using a stress-strain curve plotted on a graph. It is simpler to express the formula by

When the prestrain is affected by the prestrain mode (the ratio and direction of the maximum principal strain to the minimum principal strain) in addition to the equivalent plastic strain of the prestrain, Function equation σ _BH = g _{1 of} tensor ε
(Ε) may be used, but as a result of the present inventors' experiments and investigations on various materials and deformation modes, in general, the function σ _BH = g (ε _eq ) of the equivalent plastic strain can be used. As a result, the present inventors have conducted experiments on various materials and deformation modes, and as a result, in general, a function σ of an equivalent plastic strain has been obtained.
It became clear that the accuracy was sufficiently high even when _BH = g (ε _eq ). In particular, if the prestrain is ε _eq <0.01, σ _BH = 0
If ε _eq ≧ 0.01, using the equation σ _BH = c using the baking hardening amount c of the metal plate after 2% prestrain as a material parameter is simple and sufficient accuracy can be obtained.
σ _BH = 0 (ε _eq <0.01) and c (ε _eq ≧ 0.01) are preferred.

Next, press forming is performed using the candidate material selected in step S10, or step S22 and step S22 are performed.
A molding simulation is performed using the material characteristic data obtained in 24 [Step S30]. At this time, it is necessary that press forming defects such as breakage, necking, wrinkles, and surface distortion do not occur, but if press forming defects occur at this step or press forming defects are expected during production, Return to step S10,
It is advisable to reselect the combination of the material to be evaluated and the thickness.
If molding is possible, then the strain applied to the panel is measured [Step S32]. However, if press molding is performed, it is not easy to measure all components of the strain tensor. assuming plane isotropic material, it may be measured maximum principal strain epsilon ₁ and the minimum principal strain epsilon ₂ by scribed circle method.

In general metal materials, the elastic component of the strain is so small that it can be ignored. Therefore, the subsequent processing may be performed using the measured value as it is as the plastic strain. On the other hand, when using the molding simulation, to simulate it may be obtained the maximum principal strain epsilon ₁ and the minimum principal strain epsilon ₂ of plastic strain results, further plate plane isotropic material as in the case of performing press molding If it is necessary to use a higher degree of anisotropy than the above assumption, all the component values of the strain tensor may be obtained, and the subsequent processing may be performed. From the viewpoint of efficiency, a method of using the maximum principal strain epsilon ₁ and the minimum principal strain epsilon ₂ assumes a plate plane isotropy, simple measuring operation and the subsequent processing of the distortion, and finally sufficient precision Is preferred because

Next, the value of the sheet thickness t after press forming and the equivalent plastic strain ε _eq are calculated from the measured strain [Step S34]. The thickness t can be calculated from the initial thickness t ₀ and the thickness strain ε ₃ using t = tex (ε ₃ ). Maximum principal strain epsilon ₁ and a plate thickness strain epsilon ₃ If the minimum yield principal strain epsilon ₂ only may be obtained by ε ₃ = -ε ₁ -ε ₂ in step S32. As a matter of course, a value obtained by actually measuring the thickness of the press-formed product in step S30 or the thickness obtained from the result of the forming simulation may be directly used. The value of the equivalent plastic strain ε _eq is the ε used when σ _eq = f (ε _eq ) was obtained from the mechanical test values in step S24.
_What is necessary is _just to calculate using the formula of _eq .

Subsequently, the values of σ _eq and σ _BH for this panel are calculated using σ _eq = f (ε _eq ) and σ _BH = g (ε _eq ) obtained in steps S 24 and S 26.
0]. Further, a step S42 of calculating a dent resistance prediction evaluation index S from the obtained t, σ _eq , and σ _BH ]. here,
S is t, σ _eq, σ _BH function _{h (t, σ eq, σ} BH) with _{S = h (t, σ eq} , σ BH) is defined. As the function should be used the function was demonstrated that high correlation with the actual dent amount, the inventors of the present inventors have proceeded extensive _{studies, h (t, σ eq,} σ BH) = √ (r m +1) / 2 (σ _eq
+ Sigma _BH) type functions metal plate defined by t ^m, plate thickness,
Regardless of the amount of work hardening and bake hardening by press molding, it was found that the amount matched well with the actual dent amount.

Here, m takes a value in the range of 1.0 ≦ m ≦ 2.4 depending on the shape and fixing method of the panel. Therefore, it is preferable to use _{S = √ (r m +1)} / 2 (σ eq + σ BH) t m, √ (r m +1) / 2 (σ eq +
sigma _BH) for it goes without saying that similar effect can be obtained by using a strong another function correlated with t ^m, another form of the function h (t, σ _eq, the sigma _BH), the present invention The scope of rights is not unduly limited.

Next, the obtained value of the dent resistance prediction evaluation index S is compared with a reference value S ^* preset in accordance with the required dent resistance [Step S44]. If the value of S is smaller than S ^*, it indicates that the required dent resistance cannot be secured with the combination of the material and the plate thickness. Repeat the above procedure. On the other hand, S ≧ S ^*
Is satisfied, it is determined that the production may be performed using the combination of the material and the plate thickness.
If S ^* is large, another material or plate thickness may be evaluated from step S10 from the viewpoint of cost and formability.

[0025]

An embodiment of the present invention will be described below with reference to the drawings. From the material (steel plate) shown in Table 1, 200 mm x 2
Blanks of 00 mm and 200 mm × 125 mm were cut out, stretched to different molding heights with a cylindrical punch having a diameter of 100 mm, and subjected to equibiaxial tensile deformation of different sizes and deformation near plane strain tensile at the center of the bottom surface. FIG. 2 shows the measured values of the applied strain. This molded product was subjected to a heat treatment at 170 ° C. for 20 minutes, then fixed with a ring having a diameter of 50 mm, applied a load of 200 N using a hemispherical indenter having a radius of 25 mm, and measured the dent d after unloading. did. FIG. 3 shows the correlation between d thus obtained and the dent resistance prediction evaluation index S obtained by the present invention. Here, 2.0 was used as the value of m. S shows a high correlation with the measured value of the dent amount. Therefore, it can be seen that the dent resistance obtained by the method of the present invention can be predicted and evaluated.

On the other hand, for example, a dent depth of 0.06 mm is given.
The value of the dent evaluation index S is about 270 (MPamm^Two)
270 (MPamm^Two) The dent resistance standard
Value S ^*In the case of material A having a thickness of 0.9 mm, for example,
Ε on the panel_eqHas a molding strain exceeding 0.18
However, the dent resistance index S is equal to the reference value S.^*= 270 (MPa
mm^Two) Is smaller and the dent amount exceeds 0.06mm
However, using a material B with a thickness of 0.9 mm,
_eqIs given a molding strain of 0.03 or more,
The panel is made of ε using 0.8 mm of material C._eqIs 0.07 or less
If the above molding strain is applied, the dent resistance index S
Reference value S^*And the dent amount should be 0.06 mm or less.
I understand. As shown in the above examples, the present invention
If, for example, the selection of materials and plate thickness should be performed efficiently and quantitatively
Can be.

[0027]

[Table 1]

[0028]

As described in detail above, according to the present invention, the dent resistance can be easily predicted and evaluated easily, and the material suitable for press forming and the thickness thereof can be efficiently determined. Can be determined.

[Brief description of the drawings]

FIG. 1 is a flowchart of a proper material selection method using a dent resistance prediction evaluation method according to an embodiment of the present invention.

FIG. 2 is a diagram showing molding strain applied to a panel used in an example of the present invention.

FIG. 3 is a diagram showing a relationship between a dent resistance prediction evaluation index S obtained by using the present invention and an actually measured value d of a dent amount for a panel used in an example of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor, Terunobu Murazato 1 Kimitsu, Kimitsu-shi, Chiba F-term (reference) in Nippon Steel Corporation Kimitsu Works 2G055 AA01 AA07 AA12 EA04 EA08 FA01 2G061 AA01 AA20 AB01 CA01 CB01 DA11 EA04 EA10

Claims (7)

[Claims] An initial plate thickness is t ₀ and an equivalent plastic stress σ
The relationship between _eq and the equivalent plastic strain ε _eq is represented by σ _eq = f (ε _eq ), and the amount of curing by heat treatment for baking paint is σ _HB =
Using a metal plate represented by g (ε _eq ), the strain introduced when the metal plate is subjected to press forming is measured, and from the measured strain, the equivalent plastic strain ε _eq after press forming and the plate thickness t The equivalent plastic stress σ _eq after molding using σ _eq = f (ε _eq ) and σ _HB = g (ε _eq ) from the equivalent plastic strain ε _eq and the paint baking process Curing amount σ
A press-formed product characterized in that _HB is determined, a dent resistance evaluation index S defined by S = h (σ _eq , σ _HB · t) is calculated, and the value of the dent resistance evaluation index S is used. Method for predicting and evaluating dent resistance. 2. A method for predicting and evaluating dent resistance of a press-formed product, wherein a strain calculated by a forming simulation is used in place of an actually measured value of strain introduced by press forming in claim 1. 3. A maximum principal strain ε ₁ , a minimum principal strain ε ₂ and r when calculating an equivalent plastic strain value from the strain measured or the calculated strain according to claim 1 or 2.
Wherein using the plate plane average value r _m values _{ε eq = (r m +1)} √
_{1 / (2r m +1) (} ε 1 2 + 2r m / (r m +1) ε
_{1 ε ²} + _{ε 2 2)} dent resistance predictive evaluation methods of the press-molded product, which comprises using a. 4. The method according to claim 1, wherein σ _eq = f (ε _eq ).
Equation σ _eq = K, ε ₀ , n as material parameters
A method for predicting and evaluating dent resistance of a press-formed product, wherein K (ε ₀ + ε _eq ) ⁿ is used. 5. The method according to claim 1, wherein σ _BH = g (ε _eq ).
Σ _BH = 0 if ε _eq <0.01, and ε _eq ≧ 0.0
If 1, bake hardening amount after 2% pre-strain of the metal plate c
A method for predicting and evaluating the dent resistance of a press-formed product, characterized by using an equation σ _BH = c with を as a material parameter. 6. The dent resistance evaluation index S according to claim 1, wherein S = h (σ _eq , σ _BH , t).
0 ≦ m ≦ 2.4 using m satisfying the equation S = √ (r _m +
_{1) / 2 (σ eq +} σ BH) t m dent resistance predictive evaluation methods of the press-molded product, which comprises using a. 7. A metal plate such that a dent resistance prediction evaluation index S obtained by using the method according to claim 1 satisfies S ≧ S ^* with a predetermined reference value S ^* . A method for selecting a metal plate for press forming, wherein a type and an initial plate thickness t ₀ are selected.