JP5515521B2 - Hot press forming method for steel sheet - Google Patents

Description

  The present invention relates to a hot press forming method for a steel sheet. The product molded by this method can be preferably used for automobile frame parts and the like.

  The hot press forming method is usually a processing method in which a steel plate (material) heated to a temperature of 600 ° C. or higher is press formed with a press. Further, in the hot press molding method, in order to obtain high strength, the material heating temperature is set to the austenitizing temperature or more, and immediately after the press molding, the mold is removed (the press mold used for molding is used as a cold material as it is. There is a processing method that quenches by using a method of removing heat from a press-molded product) (refer to, for example, Patent Document 1), which is referred to herein as a direct molding method. The direct forming method is a processing method that has excellent stretch formability and stretch flange formability compared to cold press forming of a high-strength steel sheet having a tensile strength of 590 MPa or more because the material becomes highly ductile at high temperatures. It is said that. On the other hand, with regard to deep drawability, since the material temperature is partially lowered by pressing the wrinkle holding part with a mold and the deformation resistance becomes relatively high, material inflow from the wrinkle holding part is suppressed, and molding is performed. Sex is reduced. For this reason, the direct molding method is considered to be a processing method that is not good at deep drawing, and the press molding method for parts including deep drawing parts is a processing method that heats and quenches after cold press molding (so-called indirect molding). Law). Since the indirect molding method requires a cooling device separately from the press machine, it is more expensive than the direct molding method.

JP 2006-26637 A

  Looking at the background art, the conventional direct forming method has a difficulty in deep drawing at the hot press forming stage, so the production of parts including deep drawing parts must rely on the relatively high cost indirect forming method. There is no situation. That is, in the conventional direct molding method, the problem remains that drawability is a bottleneck for expanding the range of applicable parts.

The present invention has been made on the basis of knowledge obtained in the development of a hot press molding method that solves the above-mentioned problems and improves deep drawability, and the gist of the invention is as follows. is there.
(1) A workpiece made of a steel plate is heated to have a press die composed of a punch, die and die cushion from a temperature of 600 ° C. or higher, and the stroke from the start of processing (blank hold) to the bottom dead center is s. When it is put into a press machine and press-molded to make a press-molded product, it is press-molded without lubrication, and the wrinkle holding part of the workpiece is sandwiched between the die and the die cushion from the start of processing to a stroke position of 0.3 s at maximum. To suppress hold wrinkles at the initial stage of molding , and then stop after moving the die cushion at a higher speed than the die to a stroke position of at least 0.7 s , causing a material inflow between the die and the punch, and from there to operate the press to take a motion that erases wrinkles generated during molding by clamping the blank holding portion of the workpiece to the bottom dead center A hot press forming method of a steel sheet, which is characterized.
(2) The hot press forming method for a steel sheet according to (1) above, wherein the press machine has a servo die cushion mechanism as means for executing the motion.
(3) The hot-press forming method of a steel sheet according to (1) or (2) above, wherein the material is a steel sheet containing C: 0.15% or more by mass%.
(4) The hot press forming method for a steel sheet according to any one of (1) to (3), wherein the press formed product is held and cooled as it is by the press die.

  According to the present invention, in particular, the wrinkle pressing portion of the workpiece is clamped by the die and the die cushion from the start of machining (blank hold) to a stroke position of 0.3 s at the maximum (starting molding), and then at least 0.7 s. Press machine to take the motion of moving the die cushion at a higher speed than the die to the stroke position (middle stage of molding) and then stopping and then pressing the wrinkle holding part of the workpiece to the bottom dead center (later stage of molding) Since the wrinkle holding force is temporarily applied in the initial stage of molding, wrinkles (hold wrinkles) in the initial stage of molding are suppressed, and a sufficient gap is created between the die and the die cushion in the middle of molding. Inflow of material is promoted, and wrinkle holding force is applied again in the latter half of molding, so that wrinkles generated during molding are eliminated. Therefore, according to the present invention, it is possible to expand the range of applicable parts of the hot press forming method.

1 is a schematic cross-sectional view showing the motion of a press in one embodiment of the present invention. (A) is a schematic plan view and (b) is a schematic three-dimensional view showing a target shape and a molding result evaluation part of a press-formed product in the examples.

  In the present invention, a workpiece made of a steel plate is heated to have a press die composed of a punch, a die and a die cushion from a temperature of 600 ° C. or more, and the stroke from the start of processing (blank hold) to the bottom dead center is s. It is the same as the normal hot press molding method in that it is put into a press machine and press molded to form a press molded product. Here, from the viewpoint of ease of molding, the molding start temperature is preferably 650 ° C. or higher. In that case, the heating temperature is 850 ° C., taking into account the temperature drop due to the cooling from the end of heating at the normally used material plate thickness (1.0 to 3.0 mm) to the start of molding (usually takes about 10 to 15 seconds). The above is preferable. Note that the heating temperature is preferably 1050 ° C. or less from the viewpoint of preventing the crystal grains from being coarse.

  When drawing by a normal hot press forming method, if a work is sandwiched between a die and a die cushion and a wrinkle holding force is applied, the heat is removed and the material inflow becomes difficult. On the other hand, when a gap is formed between the die and the die cushion, for example, with a distance block, flange wrinkles and body wrinkles are generated. In particular, wrinkles (hold wrinkles) that occur in the initial stage of molding may be particularly difficult to erase.

On the other hand, in the present invention, press forming is performed without lubrication, and the wrinkle holding portion of the workpiece is formed by the die and the die cushion from the start of processing (blank hold) to the stroke position of 0.3 s at the maximum (in the initial forming). The material is sandwiched between the die and the punch by suppressing the holding wrinkle at the initial stage of molding and then moving the die cushion at a speed higher than that of the die to the stroke position of at least 0.7 s. flows to cause a, it was decided to operate the press to take a motion that erases wrinkles generated during molding by clamping the blank holding portion of the workpiece (in the molding late) to the bottom dead center from which . With this motion, wrinkle holding force is temporarily applied at the initial stage of molding to suppress wrinkles (hold wrinkles) at the initial stage of molding, and a sufficient gap is formed between the die and the die cushion in the middle of molding to prevent material inflow. The wrinkle pressing force is applied again in the latter half of the molding, and the wrinkles generated during the molding can be erased. Here, in normal cold forming, it is not easy to eliminate wrinkles only by a wrinkle holding force load in the latter stage of forming, but in hot press forming, this is possible because the flow stress of the work is small.

Here, if the relative distance (a / s) from the processing (blank hold) start position (origin) to the stroke position a at the initial stage of molding where the wrinkle holding force is initially applied is greater than 0.3, the material flow Since the work is cracked due to the shortage, a / s needs to be 0.3 or less. In addition, since the initial stage of molding when the wrinkle holding force is first applied is not zero, a / s is, of course, more than 0.0. If a / s is less than 0.05, it is relatively difficult to eliminate the hold wrinkles. Therefore, a / s is preferably set to 0.05 or more. In addition, from the processing (blank hold) start position (origin), the end point of the middle stage of forming the gap between the die and the die cushion (that is, the start point of the latter stage of closing the gap between the die and the die cushion and loading the wrinkle holding force) If the relative distance (b / s) to the stroke position b is less than 0.7, the workpiece is cracked due to insufficient material inflow in the later stage of molding, so b / s needs to be 0.7 or more. Further, the molding late not a zero, b / s is of course less than 1.0. Note that if b / s exceeds 0.95, it is relatively difficult to eliminate wrinkles generated in the middle of molding, so b / s is preferably 0.95 or less.

Thus, by applying the motion referred to in the present invention in a normal hot press molding method, the drawability that has been a bottleneck for expanding the range of applicable parts is improved and no longer a bottleneck.
In order to execute the motion, a press machine having a mechanism that can easily move the die cushion with high precision in accordance with the press operation is preferable. As such a mechanism, a mechanism for controlling the movement of the die cushion by a servo system, that is, a servo. There is a die cushion mechanism. Therefore, the press used in the present invention preferably has a servo die cushion mechanism as the motion execution means (present invention (2)).

  In the present invention, in order to obtain a press-molded product with higher strength, it is preferable to quench (quenify) the press-molded product immediately after press molding. When quenching, the material is preferably a steel plate having a composition containing C: 0.15% or more by mass (present invention (3)). If C is less than 0.15%, the quenching effect may be insufficient. Other elements are not particularly limited, and basic elements such as Si, Mn, P, and S may be included as appropriate, and Cr, Mo, Ti, B, and the like may be added to improve hardenability. Good. The heating temperature is preferably 850 ° C. or higher in order to sufficiently austenite the material.

As a method for carrying out the quenching, a method of cooling the press-molded product as it is held by the press mold (this invention (4)) is preferable because it is not necessary to have a separate cooling device.
The motion of the press in one embodiment of the present invention will be described with reference to FIG. The punch 1 is a fixed mold, the die 2 and the die cushion 3 are movable molds driven by a servo mechanism, and the work 5 is finally drawn by the die 2 central recess and the punch 1 at the drawing part. The wrinkle pressing portion is pinched and restrained by the peripheral edge convex portion of the die 2 and the die cushion 3. The stroke from the time when the die 2 starts to contact the workpiece 5 (molding start) to the bottom dead center is the stroke s (mm). The boundary between the initial molding stage and the middle molding stage is set at the position of stroke a (mm). According to the provisions of the present invention, a / s is more than 0.0 and 0.3 or less. Further, the boundary between the middle stage of molding and the latter stage of molding is set at the position of stroke b (mm). The servo mechanism applies a wrinkle holding force to the wrinkle holding part in the initial stage of molding, stops the die cushion 3 after moving the die cushion 3 at a higher speed than the die 2 in the middle stage of molding, and promotes the inflow of the material. It is set so that the wrinkle holding force is loaded again.

  First, as shown in FIG. 1A, the workpiece 5 is put into the press machine, and only the die 2 is immediately moved (lowered). As shown in FIG. 1B, at the initial stage of molding up to the stroke a (mm), a wrinkle pressing force is applied to suppress the hold wrinkle. Subsequently, as shown in FIG. 1C to FIG. 1D, in the middle stage of molding up to the stroke b (mm), the die cushion 3 is moved and stopped at the moving speed of the die 2 <the moving speed of the die cushion 3. . During this time, a large inflow of material occurs, and further, when the die 2 reaches the position of the stroke b (mm) and is attached to the die cushion 3, the wrinkle pressing force is regenerated, and the process proceeds to the latter stage of molding. After this, it is the same as normal drawing, and the body wrinkle disappears by the bottom dead center, and quenching is performed by heat removal from the mold as shown in FIG.

  In performing hot press forming using a steel plate (material) having the composition and thickness shown in Table 1 as a workpiece, the material is heated to 950 ° C. and put into a press machine similar to the press machine shown in FIG. Molding was started at 750 ° C., molding was performed without lubrication under the various molding conditions shown in Table 2, and quenching was performed by removing heat at the bottom dead center. However, a press-formed product 6 showing the same target shape) was obtained. About these press-molded products, the molding results at the molding result evaluation sites shown in FIG. 2 were visually evaluated. The results are shown in Table 2. From Table 2, any of cracks, body wrinkles, and flange wrinkles occurred in the comparative example, whereas none of them occurred in the inventive example (that is, OK), and the effect of the present invention was manifested.

1 punch 2 die 3 die cushion 5 work (the material is steel plate)
6 Press-molded products

Claims (4)

A workpiece made of steel plate is heated and pressed from a temperature of 600 ° C or higher into a press machine with a stamping die consisting of a punch, die and die cushion, and the stroke from the start of processing to bottom dead center is s. When forming and forming a press-molded product , press molding is performed without lubrication, and the wrinkle holding part of the workpiece is clamped between the die and the die cushion from the start of machining to a stroke position of 0.3 s at the maximum, and the initial hold is formed. suppressing Wa, then moved after stopping the die cushion at a high speed from the die to the stroke position of at least 0.7 s, the die and to cause a material flowing between said punch, wrinkles of the work from there to the bottom dead center hot pressing of a steel sheet, characterized in that to operate the press to take motion of the pressing portion by clamping erasing wrinkles generated during molding Form method.   The hot press forming method for a steel sheet according to claim 1, wherein the press machine has a servo die cushion mechanism as means for executing the motion.   The method of hot press forming a steel sheet according to claim 1 or 2, wherein the material is a steel sheet containing C: 0.15% or more by mass%.   The hot press forming method for a steel sheet according to any one of claims 1 to 3, wherein the press formed product is held and cooled as it is by the press die.

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