JP5505014B2 - Strength prediction method and strength control method for hot press molded products - Google Patents

Description

  The present invention relates to a method for predicting the strength of a molded product when a metal plate is hot press-molded and a method for controlling the strength, and is mainly used for hot press molding of structural parts of automobiles.

  In recent years, especially in the automobile industry, materials have been made thinner and higher in strength by reducing the weight of vehicle bodies due to environmental problems. In many cases, press molding is used for the production of such a component, but cracking of the material and poor dimensional accuracy are likely to occur in press molding using a highly strengthened material (metal plate).

  In order to solve such a problem, hot press molding (hot press molding) is known as a molding method capable of suppressing cracking of a material (metal plate) and poor dimensional accuracy. This technology is introduced in Non-Patent Document 1, and for example, a steel plate is heated to an austenite region of about 900 ° C. and subjected to quenching (die quench) by press cooling while being contact cooled with a mold, and the strength of the steel plate is increased. This is an enhanced molding method.

  In such hot press molding, the strength of the molded product (molded product) is determined by the temperature history (cooling speed, etc.) and processing history (processing amount, etc.) of the material at the time of molding. Therefore, it is difficult to observe the temperature history and processing history of the material. Therefore, by analyzing the temperature history and processing history of the material in advance by computer simulation and predicting the strength of the molded product based on it, the molding conditions (heating temperature of the material) that the molded product can obtain the desired strength It is effective to find the initial temperature of the mold, die quench time, etc.).

  Incidentally, as a computer simulation related to hot press molding, Patent Document 1 discloses a simulation method of a mold temperature.

  However, in the method disclosed in Patent Document 1, although the mold temperature is known, the strength of the molded product cannot be predicted, and the molding of the hot press is performed so that the molded product can obtain a desired strength. The conditions cannot be adjusted.

JP 2008-055488 A

Press Technology Vol.43 No.9 (August 2005)

  The present invention has been made in view of the above circumstances, and a strength prediction method for a hot press molded product that can accurately predict the strength of the hot press molded product when performing hot press molding of a metal plate. And a strength control method for a hot-press molded product that makes it possible to appropriately adjust the molding conditions of the hot press so that the molded product can obtain a desired strength based on the strength prediction result. It is the purpose.

  In order to solve the above problems, the present invention has the following features.

[1] A hot press molded product strength prediction method for predicting the strength of a hot press molded product by a heat transfer structure analysis program by a finite element method when hot pressing the metal plate,
Create a model for the finite element method and set hot press forming conditions, perform coupled analysis using the heat transfer structure analysis program, and heat the metal plate set as the hot press forming conditions and the coupled analysis A machining CCT curve is created based on the machining amount of the obtained metal plate, and the hardness of each element is calculated using the temperature history of the metal plate obtained by the coupled analysis and the machining CCT curve. A method for predicting the strength of a hot press-molded product, wherein the hardness is converted into strength.

[2] Hot press molding that adjusts hot press molding conditions based on the strength prediction result by the strength prediction method of the hot press molded product according to [1] so that the hot press molded product can obtain a target strength. A method for controlling the strength of a product,
It is judged whether or not the strength prediction result by the strength prediction method of the hot press molded product described in [1] satisfies the target strength. If the target strength is not satisfied, the hot press molded product is Dividing into a predetermined number of sections and selecting any changeable molding condition from among the changeable molding conditions set in advance as the molding conditions that can be changed in the hot press molding conditions. Is set for each section, and the hot press molding condition satisfying the target strength is detected by predicting the strength by the strength prediction method of the hot press molded product described in [1]. Strength control method for hot press molded products.

  The strength of the hot press molded product can be accurately predicted by the method of predicting the strength of the hot press molded product of the present invention. Also, the hot press molded product strength control method of the present invention makes it possible to appropriately adjust the hot press molding conditions so that the hot press molded product can obtain the target strength.

It is a flowchart which shows Embodiment 1 of this invention. It is a figure which shows a process CCT curve. It is a flowchart which shows Embodiment 2 of this invention. It is a figure which shows an example of division of the molded article in Embodiment 2 of this invention.

  Embodiments of the present invention are described below.

[Embodiment 1]
In Embodiment 1 of the present invention, when hot pressing a metal plate (blank), the strength of the molded product is predicted by a heat transfer structure analysis program using a finite element method, and a flow diagram is shown in FIG. In addition, the following steps (S1) to (S6) are provided.

  (S1) Create a finite element method model and set hot press molding conditions. Here, the creation of the finite element method model determines the element division and physical property values for the mold and the blank, and the hot press molding conditions are set by the heating temperature and heating time of the blank, the initial temperature of the mold. The moving speed of the mold, the die quench time, etc. are set.

  (S2) Then, coupled analysis is performed by a heat transfer structure analysis program. In addition, as a heat-transfer structure analysis program, the general purpose heat-transfer structure analysis program marketed can be used, for example.

  (S3) Based on the heating time of the blank set as the hot press molding condition in (S1) and the blank processing amount obtained in the coupled analysis of (S2), depending on the heating time and processing amount A processed CCT curve is created. The machining CCT curve will be described later.

  (S4) The hardness of each element of the molded product is calculated using the temperature history of the blank obtained in (S2) above and the processed CCT curve created in (S3) above. The method for calculating the hardness will be described later.

  (S5) The hardness of each element obtained in (S4) is converted into strength. In addition, the conversion method to intensity | strength is mentioned later.

  (S6) The intensity of each element obtained in (S5) above is mapped.

  Here, as described above, the machining CCT curve will be described with reference to FIG.

  Normally, a CCT curve (Continuous Cooling Transformation diagram) is created in a laboratory, but at that time, the heating time of the test piece is relatively short and no processing is applied to the test piece. On the other hand, in the hot press molding, the heating time of the blank is relatively long and the blank is processed. Therefore, in this embodiment, as shown in FIG. 2, a CCT curve created in the laboratory (laboratory CCT curve) is used as a CCT curve calibrated for hot press molding in accordance with the heating time difference and the processing amount difference between the two. Thus, a CCT curve with a raised transformation point (processed CCT curve) is created, and the processed CCT curve is used to calculate the hardness of the hot press molded product.

  At that time, the transformation point raising temperature ΔT is calculated by the following equation.

ΔT = T · k · Δt · (−ln (L) / a)
Where T: transformation point in the laboratory CCT curve
Δt: heating time difference
L: Processing amount (true strain amount by hot press molding)
a: Coefficient
k: coefficient

  Next, as described above, a method for calculating hardness based on the machining CCT curve will be described.

In the CCT curve (laboratory CCT curve) created in the laboratory, the cooling curve of the test piece, the transformation start and end temperatures, the transformation amount, and the hardness of the test piece (Vickers hardness Hv) are described. The CCT curve calibrated for hot press molding as described above (CCT curve with a raised transformation point (processed CCT curve)) is similarly applied to the test piece cooling curve, transformation start and end temperatures, transformation amount and test. The hardness of the piece (Vickers hardness Hv) is described. Therefore, the structure and temporary hardness (Vickers hardness Hv _tmp ) can be estimated by applying the temperature history of the blank obtained by the coupled analysis (S2) by the heat transfer structure analysis program to this machining CCT curve. The temporary hardness (Vickers hardness Hv _tmp ) estimated from this processed CCT curve is added to the hardness change amount ΔHv expressed by the following equation (the effect of the recrystallized grain size becoming smaller due to strain caused by machining, and the larger the grain size due to heating time difference The hardness of the hot-pressed product (Vickers hardness Hv _true ) can be calculated by adding the above-mentioned influence and the hardness change amount considering the influence of the change in the metal structure ratio.

Hv _true = Hv _tmp + ΔHv
ΔHv = m / (− ln (L) −Δt) ^{1 / 2n} + p · ΔT · Hv _tmp
Here, Hv _true : Hardness of hot press product (Vickers hardness)
Hv _tmp : Temporary hardness estimated from the machining CCT curve (Vickers hardness)
ΔHv: Hardness change
Δt: heating time difference
ΔT: Raising temperature of transformation point
L: Processing amount (true strain amount by hot press molding)
m: Coefficient determined according to temperature, processing amount, and material characteristics
n: Coefficient determined according to temperature, processing amount, and material characteristics
p: Coefficient determined according to temperature, processing amount, and material characteristics

  Further, as described above, a method for converting hardness to strength will be described.

  It is conventionally known that there is a correlation between hardness and strength, and that the tensile strength TS (MPa) is about three times the Vickers hardness Hv. Therefore, a large number of data in which the hardness (Vickers hardness Hv) is associated with the strength are accumulated in advance, a database is created, and a conversion formula for converting the strength from the hardness is created. If the hardness (Vickers hardness Hv) calculated based on the machining CCT curve is applied to this conversion formula, the corresponding strength can be obtained.

  As described above, in the first embodiment, the strength of the hot press molded product can be accurately predicted.

[Embodiment 2]
The second embodiment of the present invention adjusts hot press molding conditions based on the strength prediction result of the hot press molded product according to the first embodiment so that the molded product can obtain a target strength. As shown in FIG. 3, the following steps (S0) to (S11) are provided. Note that (S1) to (S6) are the same as those in the first embodiment.

  (S0) The target strength (distribution) of the molded product is set.

  (S1) Create a finite element method model and set hot press molding conditions.

  (S2) Then, coupled analysis is performed by a heat transfer structure analysis program.

  (S3) Based on the blank heating time set in (S1) above and the blank machining amount obtained in the coupled analysis in (S2) above, a machining CCT curve corresponding to the heating time and machining amount is obtained. create.

  (S4) The hardness of each element of the molded product is calculated using the temperature history of the blank obtained in (S2) above and the processed CCT curve created in (S3) above.

  (S5) The hardness of each element obtained in (S4) is converted into strength.

  (S6) The intensity of each element obtained in (S5) above is mapped.

  (S7) It is determined whether or not the intensity obtained in (S6) satisfies the target intensity set in (S0). If satisfied, the process proceeds to (S11). If not satisfied, the process proceeds to (S8).

  (S8) The blank (molded product) after molding is divided into a predetermined number of sections. For example, the molded product is partitioned as shown in FIG.

  (S9) An arbitrary change parameter is selected from among the change molding conditions (change parameters) previously determined as the molding conditions that can be changed among the molding conditions. Here, the heating time of the blank, the initial temperature of the mold, and the die quench time (mold gap) are used as the change parameters.

  (S10) The value (change value) is set for the change parameter selected in (S9) for each of the sections divided in (S8). Then, the process returns to (S2). The setting of the change value of the change parameter will be described later.

  (S11) The process is terminated assuming that the molding conditions determined to satisfy the target strength in (S7) are the optimum conditions for this hot press molding.

  Here, as described above, the setting of the change value of the change parameter will be described.

The change parameter is X, its initial value (set by (S1)) is X ₀ , the prediction strength obtained thereby is Ts ₀ , the i-th change value is X _i , and the prediction strength obtained thereby is Ts. _{When i} is set, the change value X _{i + 1} of the (i + 1) th time is set as follows.

X _{i + 1} = X _i + ΔX _{i + 1}
ΔX _{i + 1} = (X _i −X ₀ ) × | (Ts _i −Ts ₀ ) / Ts ₀ |

  By doing the above, in this Embodiment 2, it becomes possible to adjust hot press molding conditions appropriately so that a hot press molded product can obtain target intensity | strength.

Claims (2)

A hot press molded product strength prediction method for predicting the strength of a hot press molded product by executing a computer with a heat transfer structure analysis program using a finite element method when hot pressing a metal plate,
Create a model for the finite element method and set hot press molding conditions, perform coupled analysis with the heat transfer structure analysis program,
Hot press where the heating time of the test piece and the heating time of the metal plate in the CCT curve created in the laboratory where the heating time of the test piece is short and the processing is not applied to the test piece are long and the metal plate is processed calculated heating time difference between the heating time set as the host Ttopuresu molding conditions in the molding,
According to the heating time difference and the processing amount of the metal plate obtained by the coupled analysis in the hot press forming, a machining CCT curve is created by raising the transformation point of the CCT curve created in the laboratory ,
Strength prediction of a hot press molded product characterized by calculating the hardness of each element using the temperature history of the metal plate obtained by the coupled analysis and the processed CCT curve, and converting the calculated hardness into strength Method. The hot press molding condition is adjusted so that the hot press molded product can obtain the target strength based on the strength prediction result by the strength prediction method of the hot press molded product according to claim 1, and the hot executed by a computer. A method for controlling the strength of a press-formed product,
It is judged whether or not the strength prediction result by the strength prediction method of the hot press molded product according to claim 1 satisfies the target strength, and if the target strength is not satisfied, the hot press molded product is determined in advance. In addition to dividing into a number of sections, select any changeable molding condition from among the changeable molding conditions set in advance as the molding conditions that can be changed in the hot press molding conditions, and set the value of the selected changeable molding conditions. A hot press molding condition that satisfies a target strength is detected by setting the strength for each section and performing strength prediction using the strength prediction method for a hot press molded product according to claim 1. Strength control method for press-formed products.

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