JP7264090B2 - METHOD FOR MANUFACTURING STEEL PLATE FOR PRESSING, METHOD FOR MANUFACTURING PRESSED PARTS, AND METHOD FOR EVALUATING STRETCH FLANGING FORMABILITY - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a steel sheet for press use, a method for producing a pressed part, a steel sheet, and a method for evaluating stretch flanging formability.

Among pressed parts, for example, pressed parts for automobiles are mass-produced by press working using a die. In press working, the flange portion of the steel plate is accompanied by stretch-flange deformation. If cracks occur in the flange portion, the molding will be defective. On the other hand, in order to prevent cracks due to stretch flanging deformation, when the flange part is work hardened by shearing, it is reported that it is effective to pre-soften it by heat treatment before stretch flanging ( Non-Patent Document 1).

An index for evaluating such stretch flanging formability includes, for example, a hole expanding test method. The hole expansion test method includes a method using a generally used conical punch and a method using a cylindrical punch. In either method, the rate of increase in hole diameter (hole expansion rate) at the time when the hole end surface reaches crack penetration is used as an index of stretch flanging formability.
On the other hand, it is known that the deformation limit strain in the flange portion of the press part depends on the strain gradient in the vicinity of the flange portion (Patent Document 1).

As a method for suppressing cracking of the stretch flange portion, there is a means for heating the punched end face, such as those disclosed in Non-Patent Document 1 and Patent Document 2, for example. According to Non-Patent Document 1 and Patent Document 2, by heating the punched end face in an appropriate temperature range, the hole expansion ratio (λ ) can be improved.

Japanese Patent No. 4935713 JP 2019-73763 A

Plasticity and Processing: 16-172 (1975), 365-370

Conventionally, it was known that hole expansion in a relatively high strain region using a cone punch depends on the heating temperature of the sheared end face. On the other hand, it has been difficult to evaluate the effects of strain gradient on the heating range and molding after heating.
The present invention focuses on the above points, and aims to set a more appropriate heat treatment temperature and heating range according to the steel material to further suppress cracking of the stretch flange portion.

To solve the problem, one aspect of the present invention is a method for manufacturing a steel sheet for press that is subjected to press working, comprising: a shearing step of shearing at least a part of an end of a steel sheet; A heating step of heating a preset heating range from the end of at least a part of the end of the steel plate that has been subjected to heating, and the heating temperature in the heating step is set to 600 ° C. or more and less than 800 ° C.; Furthermore, by heating the steel sheet under the same heating conditions as the heating step, a ductility estimation step of estimating a change in ductility of the heated part, and based on the estimation of the ductility estimation step, when it is estimated that the ductility will be improved by heating. The heating range is set to a range of 3 mm or more from the end surface, and when it is estimated that the ductility will decrease due to heating, the heating range is set to a range of 6 mm or less from the end surface. do.

Another aspect of the present invention is a method for manufacturing a press-formed steel sheet, comprising: a shearing step of shearing at least a part of an end of a steel sheet; a heating step of heating a predetermined heating range from the end of at least a part of the end of the steel plate, the steel plate being a steel type that does not contain a retained austenite phase, and heating in the heating step The gist is that the temperature is set to 600° C. or more and less than 800° C., and the heating range is set to a range of 3 mm or more from the end surface.

Another aspect of the present invention is a method for manufacturing a press-formed steel sheet, comprising: a shearing step of shearing at least a part of an end of a steel sheet; a heating step of heating a predetermined heating range from the end of at least a part of the end of the steel plate, the steel plate being a steel type containing a retained austenite phase, and the heating temperature in the heating step is set to 600° C. or more and less than 800° C., and the heating range is set to a range within 6 mm from the end face.

ADVANTAGE OF THE INVENTION According to the aspect of this invention, it becomes possible to provide a press steel plate that can further suppress cracking of the stretch flange portion by setting a more appropriate heat treatment temperature and heating range according to the steel material.

It is an explanatory view schematically showing the height of the strain gradient due to the shape of the flange portion. It is a diagram showing the relationship between the true strain and the strain gradient of the flange end face in each of the conical and cylindrical hole expansion. FIG. 5 is an explanatory diagram showing the difference in properties in stretch flange fracture depending on the presence or absence of strain localization. It is a figure which shows the example of a process concerning embodiment based on this invention. It is a figure explaining a steel plate manufacturing process. It is a figure explaining the CAE model of a steel plate. 1 is a flow diagram outlining a method of implementing the present invention; FIG. 4 is a diagram showing changes in mechanical properties of thin steel plate materials A, B, and C due to heating. FIG. FIG. 4 is a diagram showing changes in the volume fraction of retained austenite in the thin steel plate material C due to heating. It is explanatory drawing of the definition of the heating range Rh from an end surface. It is explanatory drawing of the stretch-flange crack determination example by the localization of the strain|strain by CAE. FIG. 4 is a diagram showing the relationship between the variation values of the hole expansion rate and the plate thickness reduction rate in a non-heated material and a whole heated material. FIG. 4 is a diagram showing the dependence of the CAE-predicted value and the experimentally-measured value of the critical cylindrical hole expansion ratio of the thin steel sheet A on the heating range. FIG. 4 is a diagram showing the dependence of the CAE-predicted value and the experimentally-measured value of the critical cylindrical hole expansion ratio of the thin steel sheet B on the heating range. 4 is a graph showing the dependence of the CAE-predicted value and the experimentally-measured value of the critical cylindrical hole expansion ratio of the thin steel sheet C on the heating range. FIG.

Next, embodiments based on the present invention will be described with reference to the drawings.
"knowledge"
Regarding the above-mentioned problem, the inventors investigated the change in stretch-flange formability due to heat treatment of the stretch-flanged portion by hole expansion using a cylindrical punch, which is a forming condition with a relatively low strain gradient in various steel grades. . In the process, the inventors have found a problem that the hole expansibility using a cylindrical punch may not always be improved by the same heating method as in Non-Patent Document 1 and Patent Document 2. More specifically, it was found that under forming conditions with a relatively low strain gradient, not only the punched edge surface but also the heating range from the edge surface has a large effect. Furthermore, it became clear that there is a large difference in the desirable heat treatment range depending on the steel type. Therefore, it was found that it is practically essential to determine the optimum heat treatment conditions according to the difference in steel type and the forming conditions with different strain gradients by formability evaluation by CAE and experiments.

Here, the strain gradient near the flange portion represents the rate of change of the maximum principal strain near the flange portion with respect to the distance away from the end portion of the flange portion. Even in actual press forming, there are forming conditions with a high strain gradient and forming conditions with a low strain gradient depending on the shape of the flange portion. In the hole expansion test, hole expansion using a conical punch as shown in FIG. corresponds to molding conditions with a relatively low strain gradient. Figure 2 shows the relationship between the true strain of the hole edge and the strain gradient when expanding the hole with an initial hole diameter of 10 mm with a conical punch with a diameter of 50 mm Φ apex angle of 60 ° and with a cylindrical punch with a diameter of 40 mm Φ. For comparison, values calculated by FEM analysis are shown. The relationship between the true strain and the strain gradient does not differ depending on the steel type, and depends only on the forming conditions.
As a result of studying the cause of the above problem and earnestly studying means for solving the problem, the inventors obtained the following findings.

(Knowledge 1)
In the material in which the flange end surface is heated at 600° C. or more and less than 800° C., the outermost surface of the flange end surface changes to a soft structure by tempering, and the work hardening due to punching is almost recovered, so FIG. Such cracks from the flange end face can be effectively avoided. Therefore, it is possible to greatly improve the hole expansion ratio using a conical punch, which is a forming condition with a relatively high strain gradient.

(Knowledge 2)
On the other hand, when the material is similarly heated and the hole is expanded under the conditions using a cylindrical punch, which is a molding condition with a relatively low strain gradient, the edge heating causes, as shown in FIG. When strain localization occurs, fracture due to cracking occurs at the stretch-flange in the early stage of hole expansion. In stretch flange cracking accompanied by such localization of strain, as shown in Fig. 3(a), non-uniform plate thickness reduction along the periphery of the flange is observed, and this non-uniform plate thickness reduction is followed by As a result, cracks quickly occur on the hole edge surface.

(Knowledge 3)
The hole expansion ratio, which causes the localization of strain in FIG. .

(Knowledge 4)
By adjusting the heat treatment range, when the heating range is set to 5 mm from the flange end face, the flange end face is heated to 600 ° C. or more and less than 800 ° C. to avoid cracks from the flange end face as shown in FIG. At the same time, the ductility of the material can be increased over a wide range of 5 mm or less from the flange except for the end face. In this way, it is possible to simultaneously suppress cracking from the flange end face as shown in FIG. 3 (b) and cracking due to localization of strain as shown in FIG. It is possible to obtain a steel sheet material having remarkably excellent stretch flanging formability.

(Knowledge 5)
From the viewpoint of (Knowledge 4), for martensitic steel or the like whose ductility is improved by heating, it is desirable to set the heating range to a relatively wide range from the flange end face. On the other hand, in TRIP steel or the like, the ductility is reduced by heating due to the decomposition of the retained austenite phase, so it is desirable to set the heating range to a relatively narrow range from the flange end surface.

(Finding 6)
For stretch-flange cracking accompanied by localization of strain in FIG. For example, by calculating the stretch flanging ratio that causes fluctuations in the thickness reduction rate by FEM analysis, prediction is possible.

The present embodiment is made based on such findings.
"First Embodiment"
First, a first embodiment will be described with reference to the drawings.
(composition)
The method for manufacturing a pressed part in this embodiment includes a steel plate manufacturing process 10 and a press working process 11, as shown in FIG. As shown in FIG. 5, the steel plate manufacturing process 10 performs a shearing process 10A and a heating process 10B in this order. The steel plate manufacturing process 10 further includes a ductility estimation process 10C and a heating range determination process 10D. The steel plate manufacturing process 10 constitutes the steps of the method for manufacturing a steel plate for pressing according to the present embodiment.

<Shearing step 10A>
In the shearing step 10A, a steel sheet made of one plate material formed by rolling or other processes is trimmed into a preset blank material shape, or an opening is formed by shearing such as burring to obtain a desired shape. This is the process of obtaining a steel plate (blank material).
Here, when a steel plate is cut by shearing, the end face is more damaged than the end face produced by machining, resulting in an uneven end face state, which deteriorates the stretch flanging formability.
Also, the portion to be sheared may be only a part of the steel plate. In this case, the end portion to be heated in the heating step 10B preferably includes an end portion which is presumed to be susceptible to stretch-flange cracking when subjected to press working.

Estimation of the end (stretch flange cracking risk site) that is estimated to be prone to stretch flange cracking when press working is performed, for example, using a computer, CAE based on the press forming conditions in the desired press working Consider and identify by analysis (molding analysis). In addition, the estimation of the stretch flange crack risk region may be specified by actual pressing. Normally, the curved portion, burring portion, etc. in a plan view are stretch flange cracking risk areas. Therefore, in a region where stretch-flange forming is performed, a flange portion having a radius of curvature of a predetermined value or more by press working may be estimated as a stretch-flange crack risk site.

<Ductility estimation step 10C>
The ductility estimation step 10C estimates a change in ductility of the heated portion by heating under the same heating conditions as in the heating step 10B. Specifically, in the ductility estimation step 10C, when the target steel plate is heated at the heating temperature set in the heating step within the range of 600 ° C. or higher and lower than 800 ° C., the ductility is improved compared to before heating. or decrease in ductility. This change in ductility due to heating is determined by the steel grade.

In this embodiment, the ductility estimation step 10C is performed as part of the heating experiment step 12. In the heating experiment step 12, investigation of information on changes in mechanical properties due to heat treatment conditions of the steel material to be subjected to edge heating is carried out.
In the heating experiment step 12, a plurality of steel plates (samples) made of a steel type that may be used for press working are prepared in advance by experiment, and each steel plate is heated from the range of 600 ° C. or more to less than 800 ° C. , heat treatment is performed under a plurality of heat treatment conditions to acquire information on the ductility of each sample after heating. The acquired experimental results are stored in the database 13 .
In the heating experiment step 12 , the material properties (mechanical properties) of the steel sheet that have changed due to heating other than the ductility are also acquired by the above experiment and stored in the database 13 .

In the above heating experiment step 12, for example, investigation of changes in mechanical properties of the steel material to be subjected to edge heating due to heat treatment conditions is performed. In the heating experiment step 12, for example, each steel material is heated in a salt bath or a heating furnace under a plurality of heating temperature conditions. Properties (material properties) are obtained by testing such as a tensile test. The steel material before heating is also tested by a tensile test or the like to obtain mechanical properties. Mechanical properties are represented, for example, by stress-strain curves.
Here, the ductility may be estimated from a stretch flanging test such as a hole expanding test. For example, it is assumed that the larger the forming limit under the forming conditions satisfying the condition of formula (1) described later, the higher the ductility.

<Heating range determination step 10D>
In the heating range determination step 10D, when it is estimated that the ductility is improved by heating based on the estimation in the ductility estimation step 10C, the heating range is set to a range of 3 mm or more from the end surface. The lower limit is 50 μm. Moreover, when it is estimated that the ductility will decrease due to heating, the heating range is set to a range within 6 mm from the end surface. There is no particular upper limit.
Here, if the steel plate is a steel type that does not contain a retained austenite phase, it is confirmed that the ductility is improved by heating in the range of 600 ° C. or higher and lower than 800 ° C. Therefore, the heating range is set to a range of 3 mm or more from the end surface. set. There is no particular upper limit. However, if the heating range is too wide, the material strength (tensile strength) is softened, and the fatigue properties of the part may deteriorate. Therefore, for example, the heating range is preferably within 10 mm from the end face.

At this time, in the case of manufacturing a steel sheet for pressing made of a steel type that does not contain a retained austenite phase, the steel sheet for pressing is manufactured using a steel sheet that does not contain a retained austenite phase.
In addition, when the steel plate is a steel type containing a retained austenite phase such as TRIP steel, it has been confirmed that the ductility decreases due to heating in the range of 600 ° C. or higher and lower than 800 ° C. Therefore, the heating range is set to within 6 mm from the end surface. set in the range of There is no particular lower limit, for example, 500 μm or more.
At this time, in the case of manufacturing a steel sheet for pressing made of a steel type containing a retained austenite phase, the steel sheet for pressing is manufactured using a steel sheet containing a retained austenite phase.

<Heating step 10B>
The heating step 10B performs a process of heating the end portion of the steel plate after the shearing step 10A.
The heating range to be heated in the heating step 10B is the end surface of the steel plate and the end portion of the steel plate in the vicinity thereof. In the heating step 10B, it is not necessary to heat the entire periphery of the steel plate end portion, and at least the steel plate end portion including the stretch flange cracking risk site identified by the estimation processing unit may be heated.
The heating range is appropriately selected from the heating range determined in the heating range determining step 10D.
The heating temperature is appropriately selected from the range of 600°C or higher and lower than 800°C.
Heating of the edge of the steel plate is performed by a laser, induction heating, or the like that can locally heat the steel plate.
The heating rate may be any rate from the viewpoint of improving the stretch flanging formability, but when heating is performed in the production process, it is desirable from the viewpoint of mass production to be 10° C./sec or more. This is not the case if mass productivity is not an issue. In addition, rapid heating is preferable for the heating rate at the time of heating.
As the heating condition in the heating step 10B, after it is estimated that the temperature of the end surface of the steel sheet reaches the target heating temperature in the heating process, the heating state may be maintained for a certain period of time. A long holding time leads to a decrease in production efficiency, so the holding time is preferably 5 minutes or less. More preferably, the holding time is 1 minute or less.

<Press processing step 11>
The press working step 11 is a step of a press part manufacturing method in which the steel plate manufactured by the method for manufacturing a steel plate for press is press-worked to manufacture a pressed part.
The press working of the present embodiment is a step of applying cold press working including stretch flanging to the steel sheet for press manufactured in the steel sheet manufacturing process 10 to form a pressed part having a desired shape.
In cold press working, a steel sheet is formed into a pressed part having a desired shape by one or more stages of press molding.
The term "cold press working" as used herein refers to press forming without heating the steel sheet during press working. It refers to press working in press molding.
The pressed part having the desired shape manufactured in the pressing step 11 may not be the final molded product (final product shape).

(Other processes)
Between the steel sheet manufacturing process 10 and the press working process 11 or as a pre-process of the steel sheet manufacturing process 10, a process related to evaluation of formability, a process of adjusting the heating conditions in the steel sheet manufacturing process 10, etc. good.

Next, a process example will be described.
<Evaluation step 14 for stretch flanging formability>
This evaluation step 14 is a step of a method for evaluating the stretch flanging formability of the steel sheet for press manufactured in the steel sheet manufacturing process 10 .
This evaluation process 14 has a molding analysis process and an evaluation determination process.

[Molding analysis process]
In the forming analysis process, the CAE model of the steel plate for press is set by changing the material properties of the area corresponding to the heating range to the mechanical properties after being changed by heating at the heat treatment temperature. Then, using the CAE model of the steel plate after the change, a forming analysis simulating press forming is performed. The forming analysis step calculates the amount of stretch flange forming at the edge of the steel sheet in the forming analysis.

[Evaluation judgment step]
In the evaluation determination step, the stretch-flangeability is evaluated based on the amount of stretch-flangeability calculated in the forming analysis step.
For example, in the forming analysis process, by changing the forming analysis conditions of the forming analysis in various ways, the amount of flange forming under the forming analysis conditions in which the strain set in advance at the edge of the flange portion is localized in the direction along the edge Calculate Then, in the evaluation determination step, as an evaluation of the stretch flanging formability of the press steel plate manufactured in the steel plate manufacturing step 10, the amount of flange forming that causes localization of the thickness reduction, which is obtained by the forming analysis step, is is predicted as the limit amount of stretch flanging.

Then, based on the press forming conditions of the press forming in the press working step 11, when the amount of stretch flanging obtained from the forming analysis is larger than the amount of flanging at which the strain localization obtained above occurs, By repeating the evaluation step 14 by changing the steel type, the range of partial heating, and the heating temperature set as the specifications of the steel plate manufacturing process 10, it is predicted that stretch flange cracking will not occur in press working. , determine the specifications of the steel sheet manufacturing process 10 .
At this time, in conjunction with the above change, or instead of the above change, by changing the forming conditions in the press working step 11, the press working process to the specifications that are expected to prevent stretch flange cracking from occurring in the press working. 11 specification may be changed.

Alternatively, as another example, the forming analysis is performed using the forming conditions of the forming analysis that simulates press forming in the forming analysis step as the forming conditions of the press working step 11, and the amount of stretch flanging at the edge of the steel plate is calculated. do. Then, it is evaluated whether or not the calculated stretch-flanging amount exceeds the forming limit of the stretch-flanging amount obtained in advance. When the forming limit of the stretch flanging amount obtained in advance is exceeded, the heating range and the heating temperature are changed as in the above example, and the evaluation step 14 is repeated.

[CAE model of steel plate]
Here, a CAE model of the steel plate for press will be described.
To create a CAE model, first, a steel plate model is expressed as a mesh model divided into a plurality of meshes (shell elements) as shown in FIG. Enter the available mechanical property data. Next, the mechanical properties of each mesh corresponding to the heating range Rh set in the specifications of the steel sheet manufacturing process 10 are changed to the mechanical properties after heating at the heating temperature. The mechanical properties after heating at the heating temperature are determined by referring to the data obtained in the heating experiment step 12 and stored in the database 13 .

[Regarding localization of strain set in advance at the edge of the flange]
Here, the localization of strain set in advance at the edge of the flange means that at the end of the stretch flange, in the direction along the end face of the end, the variation in the plate thickness reduction rate is large, and the strain is locally It refers to a state in which there is a risk of occurrence of stretch flange cracking (see FIG. 3(a)). The plate thickness reduction rate is the plate at the position where the plate thickness reduction amount is the largest, relative to the plate thickness at the position where the plate thickness reduction amount is the smallest along the edge at the end of one stretch flange. thickness ratio. For example, it is assumed that a preset strain localization occurs when the plate thickness reduction rate is 5% or more (see Examples).

According to the above-described treatment, it is possible to optimize the design of the heat treatment conditions at the steel plate edge portion that will become the shear flange portion. For example, depending on the assumed heating range and heating temperature of the steel plate flange portion, it is possible to evaluate the stretch flange formability, especially in the low strain gradient region. Similarly, it becomes possible to design the heating range and heating temperature of the steel plate flange portion necessary to obtain sufficient stretch flanging formability for the required forming conditions.
According to the heating method of the present embodiment, it is possible to set optimum heating conditions according to the steel type of the steel plate.
Especially under forming conditions with a relatively low strain gradient, it is possible to improve the stretch flanging formability of the press steel plate. Here, the molding condition with a relatively low strain gradient means a lower molding condition with a corresponding strain gradient when the end face strain is the same compared to the cylindrical hole expansion as shown in FIG. are doing.
Molding conditions that satisfy the following formula (1) are desirable.

The true strain is the strain obtained in the direction along the flange edge, and the strain gradient is the value obtained in the direction away from the flange edge (for example, the planar direction perpendicular to the edge).
That is, the present embodiment is particularly effective when pressed parts are produced by pressing under forming conditions with a relatively low strain gradient.
Then, by using the steel plate manufactured by the method for manufacturing a steel plate for press use of the present embodiment for press working, it is possible to form a press that cannot be formed with a blank material manufactured by a sheared end surface unheated material or a conventional sheared end surface heating method. Molded products can be produced.

"Example of embodiment"
An example of an embodiment will now be described with reference to the drawings.
FIG. 7 is a flow diagram outlining an embodiment in accordance with the present invention.
Step 100 is an experimental step, in which the change in mechanical properties of the steel material to be subjected to end face heating is investigated depending on the heat treatment conditions. In step 100, the steel material is actually heated in a salt bath or a heating furnace, and changes in mechanical properties are examined by a tensile test or the like. ) is measured and stored in the database 13 .

Step 110 is an analysis step, in which a CAE model is produced in which mechanical properties are changed according to the steel type, heating temperature, and heating range. Here, based on the data in the database 13 obtained in step 100, the material properties (mechanical properties) are obtained in step 100 by the heating range and the heating temperature range according to the heating conditions of the actual heating or the assumed specifications. A modified CAE model (see FIG. 6) is created. This CAE model also includes a molding process simulation model, and ideally includes a molding simulation model under various molding conditions with different strain gradients.

Step 120 is an analysis step, in which forming analysis for evaluating formability is performed using the CAE model created in step 110 .
In the process of step 120, by performing a forming test of the same model by CAE, for cracks due to localized strain as shown in FIG. By doing so, the stretch-flange limit strain is predicted. In the process of step 120, for example, based on the measured value of the hole expansion ratio in step 100, by simultaneously evaluating the stretch flange limit strain by a conventional prediction method such as Patent Document 1, FIG. It is also possible to consider cracks from the flange end face such as

Step 130 is an analysis process, and comparison with the formability in step 120 is performed with the target press-molded product as required molding conditions. Calculate the strain gradient and stretch flange strain required by the required press parts here and compare with the evaluation of stretch flange propriety in step 120. If the formability in step 120 is insufficient, go back to step 110 and review the steel grade, heating temperature, and range to re-determine the appropriate steel grade and heating conditions, and perform the evaluation in steps 120 and 130 again. .

Step 140 is an analysis step to determine appropriate steel grades and heating conditions.
Specifically, as described above, steps 110 to 130 are repeated to determine provisional heating conditions that satisfy the stretch flanging formability required for the required molding conditions.
Step 150 is an analysis step in which, based on the provisional heating conditions determined in step 140, realistically possible heating means and conditions are determined. For example, when laser heating is used, it is possible to adjust the scanning speed, scanning range, and output of the laser according to the required reaching temperature and heat-affected range.

Step 160 is an experimental step, for example, by actually performing a heating and forming test of the flange portion using a small test piece such as an expanded hole test piece, the effect of heating on stretch flange formability is experimentally tested. confirm. Here, if the expected stretch flanging formability is not achieved in steps 110 to 140, it is possible to return to step 150 and examine the heating method and heat treatment conditions.
Step 170 is an experimental process, and by actually applying the heating method under the heating conditions whose effect was confirmed in step 160 to the flange portion heating and press forming of a large blank material, a large press-formed product is actually manufactured. It is a process to do. Step 170 is intended for application of partial heating to actual mass production processes. Prior to step 170, by using the optimal design method for the heat treatment conditions in the sheared flange portion of the steel plate consisting of steps 100 to 160 in this patent, the selection of heating conditions and the feasibility of forming the flange portion can be evaluated more cheaply and quickly. It is possible to

In step 160 or step 170, it is desirable to set the heating range from the flange end surface to at least 3 mm or more and heat to 600° C. or more and less than 800° C. for steel types that do not contain a retained austenite phase. On the other hand, it is desirable that the steel type containing a retained austenite phase is heated to 600° C. or more and less than 800° C. within a maximum heating range of 6 mm from the flange end face. This is because the above findings (5) and the heating conditions required to increase the stretch flangeability limit at a relatively low strain gradient, which was found from investigations including Examples.

Examples are described below. In the examples, the yield strength (YS), tensile strength (TS), total elongation, thickness shown in Table 1, and the hole expansion ratio λ of the Japan Iron and Steel Federation standard (initial hole diameter 10 mm, clearance 12%, punch apex angle values at 60°) were used.
Here, the thin steel plate material A is a DP steel having a material structure of martensite and ferrite, and the thin steel plate material B is a full martensitic steel having a martensite single phase, which are steel types whose ductility is improved by heating. On the other hand, the thin steel plate material C is a TRIP steel containing retained austenite in the material structure, and is a steel type in which the retained austenite is decomposed by heating and the TRIP effect is lost, resulting in a decrease in ductility.

Changes in mechanical properties due to heat treatment were investigated based on step 100 of the above embodiment. However, a heating method using a salt bath was used for the heat treatment, and the temperature was raised over 15 to 25 seconds, and the maximum temperature reached was defined as the heating temperature.
FIG. 8 shows changes in mechanical properties of the steel sheets A, B, and C due to heating. On the other hand, FIG. 9 shows changes in the volume fraction of retained austenite in the steel sheet C due to heating.
In the steel sheets A and B, the tempering by heating reduced the strength and improved various ductility including the total elongation. On the other hand, in the steel sheet C, the elongation decreased as the retained austenite disappeared.
As described above, the ductility of a steel type that does not contain a retained austenite phase is generally improved by heating, whereas the ductility of a steel type that contains a retained austenite phase is generally decreased by heating.

Next, based on step 110 of the embodiment, a CAE model was produced in which the mechanical properties were changed according to the steel type, heating temperature, and range. The material data for the CAE model was created based on the SS curve (mechanical properties) in the rolling direction of the sheet material using an isotropic hardening model.
In the CAE model of the example, a thin steel plate material having a hole portion with a diameter of 10 mm was expanded by a cylindrical punch having a diameter of 40 mmΦ and a shoulder radius of 5R. The reason why the cylindrical hole expansion was selected as an example here is that it represents the molding conditions when the strain gradient is relatively low. However, in view of the practical calculation load, the model of the thin steel plate is a shell element, and the mesh size in the diameter direction of the hole is unified at 1 mm. In the CAE model, in order to represent changes in material properties due to heating, only the area defined as the heating range Rh as shown in FIG. was used. In the examples, a plurality of CAE models of thin steel sheets were produced by changing the heating range Rh from 0 mm to 10 mm.

Furthermore, based on step 120, the FEM analysis of the stretch-flange formability of the produced CAE model was performed. In this example, for stretch flange cracking accompanied by localization of strain, a change of 5% or more in the entire circumferential region of the thickness reduction rate τ (%) in the circumferential direction of the hole edge surface (change in thickness reduction rate ) as a reference, cracking was determined by localization of strain on CAE, and the limit hole expansion rate was recorded. FIG. 11 shows an explanatory diagram of crack determination by localization of strain on CAE. According to this method, it is possible to determine the localization of the plate thickness reduction region, that is, the localization of the strain that can occur on a part of the circumference at a certain hole expansion rate as the hole expansion deformation progresses.

FIG. 12 is a diagram illustrating the relationship between the hole expansion ratio on CAE and the fluctuation value of the plate thickness reduction ratio in the circumferential direction for a non-heated material and a whole heated material. The x mark in the figure indicates the timing at which the fracture of the stretch flange portion occurred in the actual test piece. As shown in this figure, when the fluctuation value of the plate thickness reduction rate in the circumferential direction exceeds 5%, the same value increases rapidly, leading to rapid destruction. However, this is not the case for the non-heated material of steel type C, because it fractured before localization of strain occurred.
Thus, it can be seen that the case where the fluctuation value of the plate thickness reduction rate in the circumferential direction is 5% can be used as the evaluation value of the stretch flanging limit.

Next, step 130 was omitted because the target large blank material does not exist in the example. Next, in step 140, appropriate steel grades and heating conditions were determined. In this example, the above three steel types A, B, and C were used in order to show the difference due to the steel type, and the heating range was varied in order to show the difference due to the heating range.
As the heating method in step 150, local annealing with a laser is used as a method of heating a relatively short heating range of 2 mm or 5 mm from the end face, and total annealing with a salt bath is used as a method of heating a wide heating range covering the entire blank. decided. The heating temperature was adjusted so that the target heating range reached about 700.degree. The actually achieved heating temperature and range were confirmed by temperature measurement by measuring the amount of reflected laser light simultaneously with heating on the surface of the plate material.

Next, based on step 160, confirmation of formability in a small test piece is performed by CAE by hole expansion with a cylindrical punch with a diameter of 40 mmΦ and a shoulder radius of 5R using a thin steel plate material having a hole made by shearing with a diameter of 10 mm. It was performed with the same mold shape as After shearing, the steel plate was either not heated or heated to reach 700° C. in the 2 mm, 5 mm region from the hole edge, or the entirety by the method determined in step 150, and then subjected to the forming test. . The time when the crack penetrated the hole edge face was recorded as the critical hole expansion ratio as a crack determination in the experiment.
Finally, by comparing the heating range and the limit hole expansion rate confirmed by the experiment with the analysis by CAE, the effect of the analysis method is confirmed, and the optimum heat treatment range according to the change in ductility of the steel plate due to heating. confirmed that they are different.

13, 14, and 15 show the limit hole expansion ratio in hole expansion by a cylindrical punch with a diameter of 40 mmΦ and a shoulder radius of 5R for thin steel plate materials A, B, and C, respectively, having a hole with a diameter of 10 mm. , CAE predictions and experimental measurements are plotted against the heating range. However, since the thin steel plate material B showed cracks without localization of strain under the condition of no heating, it is indicated to that effect. Regarding the correspondence between the heating range and the heating method in the experimental values, plots with a heating range of about 2 mm and 5 mm are due to laser heating, and plots with a heating range of 10 mm or more are due to whole heating by salt bath heating. is.

13 and 14, in the thin steel plate materials A and B, the larger the heating range, the greater the increase in the cylindrical hole expansion ratio relative to the unheated material. This is because the ductility of the thin steel sheets A and B is improved by heating at 700 ° C. Therefore, the wider the heating range, the more the overall ductility is improved within 5 mm from the end face, which is related to the localization of strain. It's for.
Furthermore, the dependence of the critical hole expansion rate on such a heating range is accurately predicted by CAE according to this patent, indicating that prior examination of the heating range by CAE is effective for designing heating conditions. Furthermore, based on the experiments and CAE of this example, the specific heating range recommended is at least 3 mm or more, ideally 6 mm or more, for steel grades whose ductility is improved by heating. It is shown.

On the other hand, as is clear from the example of FIG. 15, in the thin steel plate material C, cracking occurs from the sheared edge of the hole edge before localization of strain occurs in the non-heated material, so the critical hole expansion ratio is low. On the other hand, in the heating material, since cracking from the sheared edge of the hole edge can be avoided, cracking occurred due to localized strain in all cases. As is clear from the experimentally measured values and CAE calculated values, the wider the heating range, the more the cylindrical hole expansion ratio tended to decrease, except for the non-heated material. This is because the ductility of the thin steel plate material C is reduced by heating at 700°C, so the wider the heating range, the lower the overall ductility within 5 mm from the end face, which is related to the localization of strain. be.

Furthermore, based on the experiments and CAE of this example, the specific heating range is recommended to be within at least 6 mm, ideally within 3 mm, for steel grades whose ductility is reduced by heating. It is shown. However, when no heating is performed, cracking of the sheared end face is not suppressed by heating, so it can be said that at least the outermost surface of the punched end face needs to be thermally affected by heating.
As shown in this example, by appropriately controlling the heating range, it became possible to significantly improve the stretch flanging formability as compared with the case where the heating range was not considered.

This is because, compared to conventional ultra-high-tensile steel materials and thin steel sheet materials made from their heating materials, the forming conditions in a region with a relatively low strain gradient are produced by the optimal design method of heat treatment conditions and heating method of this patent. These results indicate that the steel plate blank material with the high strength has remarkably excellent stretch-flange formability. Therefore, by using the optimum design method for heat treatment conditions and the heating method of this patent, it has become possible to produce press-formed products that could not be formed by conventional ultra-high tensile strength steel or partial heating methods.

10 Steel plate manufacturing process 10A Shearing process 10B Heating process 10C Ductility estimation process 10D Heating range determination process 11 Press working process 12 Heating experiment process 13 Database 14 Evaluation process

Claims (5)

A method for manufacturing a steel sheet for press that is subjected to press working,
A shearing step of shearing at least a part of the end of the steel plate;
A heating step of heating a preset heating range from at least one of the sheared ends of the steel plate with a heating device ;
with
The heating temperature in the heating step is set to 600° C. or more and less than 800° C.,
Furthermore, a ductility estimation step of estimating a change in ductility of the heated portion by heating the steel sheet under the same heating conditions as in the heating step;
Based on the estimation of the ductility estimation step, when it is estimated that the ductility will be improved by heating , the heating range to be heated in the heating step is set to a range of 3 mm or more from the end surface from the position of the end surface, and the ductility is reduced by heating. If it is estimated that, the heating range to be heated in the heating step , from the position of the end surface, a heating range determination step of within 6 mm from the end surface ;
has
The end portion heated in the heating step is a portion that undergoes stretch flange forming in the press working,
A method for manufacturing a steel plate for press, characterized by: A method for manufacturing pressed parts, comprising subjecting a steel plate manufactured by the method for manufacturing a steel plate for pressing according to claim 1 to press working to manufacture pressed parts. A method for evaluating stretch flanging formability of a steel sheet manufactured by the method for manufacturing a steel sheet for press use according to claim 1 ,
In the CAE model of the steel plate, set the mechanical properties of the material after changing by heating at the heating temperature in the heating process as the mechanical properties of the model region corresponding to the above heating range, and set the CAE model of the steel plate. , has an analysis step of performing a forming analysis simulating press forming and calculating the amount of stretch flange forming at the edge of the steel sheet in the forming analysis,
A method for evaluating stretch-flangeability for evaluating stretch-flangeability based on the amount of stretch-flangeability calculated in the analysis step. The evaluation of the stretch flanging formability is performed by calculating the amount of flanging when a preset strain localization occurs along the edge of the flange portion, and using the calculated amount of flanging, stretch flanging. 4. The method for evaluating stretch flange formability according to claim 3 , wherein the limit is predicted. A method for manufacturing a pressed part by press forming a steel plate to manufacture a pressed part,
Performing a forming analysis simulating the press forming, the amount of stretch flanging required for the press part is calculated,
Selection of the steel type of the steel plate, selection of the heating range, and A method of manufacturing a pressed part, characterized in that at least one selection of heating temperature is performed.

Images