JP5952881B2 - Press molded product manufacturing equipment - Google Patents

Description

In the field of manufacturing a thin steel sheet molded product mainly applied to an automobile body, the present invention heats a steel sheet (blank) as a raw material to an austenite temperature (Ac ₃ transformation point) or higher, and then press-forms the predetermined steel sheet. This is related to the production of press-molded products that can be hardened at the same time as shaping to obtain the desired strength, and particularly good in productivity without causing breakage or cracking during press molding. The present invention relates to the manufacture of a press-molded product that can realize proper molding.

  From the viewpoint of protecting the global environment, it is strongly desired to reduce the weight of automobiles for the purpose of reducing fuel consumption. When steel plates are used for the parts that make up vehicles, high-strength steel plates are used. Weight reduction is achieved by reducing the plate thickness. On the other hand, in order to improve the collision safety of automobiles, automobile parts such as pillars are required to have higher strength, and there is an increasing need for ultra-high strength steel sheets having higher tensile strength.

  However, when the strength of the thin steel plate is further increased, the elongation EL and the r value (Rankford value) are lowered, and the press formability and the shape freezeability are deteriorated.

Under these circumstances, in order to realize a high-strength automotive structural component, a hot pressing method (so-called “hot pressing method”) that simultaneously improves the strength of the component by press molding and quenching has been proposed (for example, Patent Document 1). In this technology, the steel sheet is heated to an austenite (γ) region above the Ac ₃ transformation point and hot pressed, and the steel sheet is simultaneously quenched by bringing it into contact with a normal temperature mold during press forming. This is a method for realizing high strength.

  According to such a hot pressing method, since the molding is performed in a low strength state, the spring back is reduced (the shape freezing property is good), and the strength of a tensile strength of 1500 MPa class is obtained by rapid cooling. Such a hot pressing method is called by various names such as a hot forming method, a hot stamping method, a hot stamp method, and a die quench method in addition to the hot pressing method.

  FIG. 1 is a schematic explanatory view showing a mold configuration for carrying out hot press molding as described above (hereinafter sometimes referred to as “hot press”). In FIG. Die, 3 is a blank holder, 4 is a steel plate (blank), BHF is a crease pressing force, rp is a punch shoulder radius, rd is a die shoulder radius, and CL is a punch / die clearance. Of these components, the punch 1 and the die 2 have passages 1a and 2a through which a cooling medium (for example, water) can pass, and the cooling medium is allowed to pass through the passages. These members are configured to be cooled.

When hot pressing (for example, hot deep drawing) using such a mold, molding is started in a state where the blank (steel plate) 4 is heated to the Ac ₃ transformation point or more and softened. That is, the steel plate 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, and the steel plate 4 is pushed into the hole of the die 2 by the punch 1 to correspond to the outer shape of the punch 1 while reducing the outer diameter of the steel plate 4. Mold into shape. Further, by cooling the punch and the die in parallel with the forming, heat is removed from the steel plate 4 to the mold (punch and die) and the bottom dead center of the forming (when the tip of the punch is located at the uppermost part: FIG. In the state shown in FIG. 1, the material is further quenched by holding and cooling. By carrying out such a molding method, it is possible to obtain a 1500 MPa class molded product with good dimensional accuracy and to reduce the molding load compared to the case of molding parts of the same strength class in the cold. The capacity of the can be small.

In the conventional hot press, the steel sheet is press-cooled in the austenite region above the Ac ₃ transformation point (for example, around 900 ° C.), so that the contact portion with the mold (punch and die) is not in contact with the portion. The difference in temperature is likely to occur, and strain concentrates on the relatively high temperature part.For example, in deep drawing, the shrink flange is cooled and does not shrink. It becomes difficult.

  Moreover, in the hot press, the press is usually performed in the vicinity of 700 to 900 ° C., and quenching is performed in the mold. Therefore, it is necessary to hold for a certain period of time at the bottom dead center of molding (when the tip of the punch is positioned at the top) Productivity is worse compared to cold pressing.

  For these reasons, various techniques for improving productivity have been proposed. For example, Patent Document 2 discloses that the wrinkle pressing portion (blank holder 3 shown in FIG. 1) is easily shrunk and a lubricant supply port is provided in the mold so that the steel plate can easily flow into the vertical wall portion. Techniques for molding while supplying a glass have been proposed. However, this technique cannot solve the fundamental problem that not only the structure of the mold is complicated but also a temperature difference is likely to occur.

  Further, Patent Document 3 proposes a processing method in which a part that becomes high in temperature is sequentially processed and a part in which the plate thickness is thin is cooled while forming. However, even in this technique, the structure of the mold is complicated, and in the case of deep drawing, for example, it is difficult to maintain the temperature of the shrink flange portion at a high temperature.

  Further, Patent Document 4 proposes a method of forming while controlling the displacement of the wrinkle presser according to the plate thickness + clearance. This technique is effective when the wrinkle presser is a uniform contraction flange, such as a cylindrical deep drawing. However, in the case of complex molding, wrinkle-occurring parts and non-wrinkle-occurring parts are distributed, and the contact surface pressure at the wave apex part (upper and lower parts of the unevenness) of the wrinkled part increases. As a result, the temperature drop is increased, resulting in an intensity distribution. As a result, an intensity distribution is generated, the inflow of the blank into the vertical wall becomes unstable, and the deep drawability is deteriorated.

By the way, when a steel plate is heated to an austenite region at an Ac ₃ transformation point or higher (eg, 900 ° C.), it is exposed to the atmosphere for several seconds when moving from a heating furnace to a press forming machine, and the steel plate surface is oxidized. A layer (scale) will be formed. This scale causes peeling during press molding and causes press wrinkles. Moreover, since the presence of such a scale deteriorates the paintability of the corrosion-resistant coating film, it is necessary to remove the scale by peening or the like after the press cooling.

  As measures to avoid inconvenience due to scale formation, surface-treated steel sheets such as aluminum plating, galvanization, and alloyed hot dip galvanization are also used for press forming materials (blanks), but by surface treatment, In addition to the increase in cost, there is a disadvantage that the required time becomes longer in the heating stage (cannot be rapidly heated because of plating retention and alloying). Further, it is conceivable to control the atmosphere in the heating furnace or around the press molding machine so as not to generate scale, but the apparatus becomes large and unrealistic.

JP 2002-102980 A JP 2007-75835 A JP 2006-192480 A JP 2005-297042 A

  The present invention has been made in view of the above circumstances, and the purpose thereof is deep drawing without complicating the mold and without causing inconvenience due to the formation of a scale on the surface of the steel sheet if necessary. It is an object of the present invention to provide a useful apparatus for producing a press-molded product having good moldability to the extent possible.

The apparatus for producing a press-formed product of the present invention that has achieved the above object includes a heating furnace that heats a thin steel plate to a temperature equal to or higher than the Ac ₃ transformation point, and a critical cooling rate to a temperature of at least 600 ° C. The temperature difference in the thin steel sheet is within 200 ° C. at the stage when the cooling means for cooling at the above speed and the thin steel sheet after cooling starts press forming and reaches 1/3 of the forming height. Thus, a press molding machine for press molding is provided.

  When press-molding using the apparatus of the present invention, the molding start temperature may be configured to start molding at a temperature higher than the martensite transformation start temperature Ms, but is not higher than the martensite transformation start temperature Ms. It may be configured to start molding at a temperature of. In particular, in the case where the forming is started at a temperature equal to or lower than the martensitic transformation start temperature Ms, there is no inconvenience due to the scale being formed on the steel plate surface.

  In addition, the apparatus of the present invention is particularly effective in the case of drawing using a crease presser, and even when applied to such a molding method, it does not have a complicated mold configuration, and breaks and cracks occur. And good moldability can be secured.

According to the present invention, after heating the thin steel plate to a temperature equal to or higher than the Ac ₃ transformation point, the thin steel plate is cooled to a temperature of at least 600 ° C. at a speed equal to or higher than the critical cooling rate, and then press forming is started. Since it was formed so that the temperature difference of the thin steel sheet was within 200 ° C. at the stage when it reached 1/3 of the forming height from the start, without causing breakage or cracking during forming Good molding became possible.

It is a schematic explanatory drawing which shows the metal mold | die structure for implementing hot press molding. It is explanatory drawing which shows the state which reached the shaping | molding limit at an early stage.

When the present inventors press a thin steel sheet after heating it to a temperature equal to or higher than the Ac ₃ transformation point, they produce a press-formed product having good formability with high productivity without causing breakage or cracking at the time of forming. Therefore, we examined from various angles. As a result, after heating the thin steel sheet to a temperature equal to or higher than the Ac ₃ transformation point, the temperature difference of the thin steel sheet is within 200 ° C. when the forming height reaches 1/3 of the forming height. As a result, it was found that good moldability can be secured by molding, and the present invention was completed.

  In the present invention, the “molding height” means the height after press molding. In the present invention, the temperature difference in the thin steel plate is 200 at the stage from the start of press forming to 1/3 of the forming height (this stage may be referred to as “the initial stage of forming”). Good moldability can be secured as long as the temperature is within the range of ° C. However, depending on the molding conditions (cooling conditions), the temperature difference may become wider thereafter. That is, as will be described later, the means for controlling the temperature difference within 200 ° C. includes “when the temperature difference acts to be small (for example, when the mold is heated from the beginning)” and “the temperature difference is There is a case of acting so as to increase (for example, when a molding speed is increased). In the former case, there is no problem because the temperature difference is small from the early stage of molding to the end of molding, but in the latter case, even if there is no temperature difference in the early stage of molding (within 200 ° C.), In some cases, the temperature difference becomes large (for example, when the molding speed is first increased and later decreased). Even when such a method is employed, it is preferable to ensure the temperature difference as described above at the final stage. However, even in such a case, if the temperature difference within 350 ° C. is secured in the final stage, good moldability is exhibited.

  With the mold configuration shown in FIG. 1, for example, when deep drawing is performed, a portion (die 2 and blank holder 3) corresponding to a blank wrinkle pressing portion (blank holder 3 shown in FIG. 1). ) Is lower in temperature than other blank parts. In such a state, a temperature difference is likely to occur in the blank (steel plate).

The inventors first heated a steel plate having the chemical composition shown in Table 1 below to 900 ° C. (Ac ₃ transformation point of this steel plate: 830 ° C., martensite transformation start temperature Ms: 405 ° C.), A cylindrical drawing molding experiment was performed using the mold shown in FIG. 1 (mold temperature: 20 ° C.) according to the procedure described above (see the examples below for other detailed conditions), and molding started at 800 ° C. In this case, the molding limit was reached early, but if molding was started at 600 ° C. (quick cooling to 600 ° C. above the critical cooling rate), good moldability was achieved, and deep-draw molding to molding bottom dead center was achieved. It turns out that you can.

  The cause of the above phenomenon was investigated by numerical analysis reproduced by the axis target model. As a result, when forming is started at 800 ° C., the high temperature portion of the steel sheet is 780 ° C. at the stage where the forming height has reached 1/3, and 540 in the low temperature portion (the portion corresponding to the wrinkle pressing portion). The temperature difference was 240 ° C. On the other hand, when forming is started at 600 ° C., the high temperature portion of the steel sheet is 580 ° C. at the stage where the forming height has reached 1/3, and in the low temperature portion (the portion corresponding to the wrinkle pressing portion). The temperature difference was 420 ° C., and the temperature difference was 160 ° C.

  Moreover, when the same steel plate as above was used and the mold temperature was set to 20 ° C. and 600 ° C., the state at the time of forming with the forming start temperature set to 800 ° C. and 750 ° C. was also investigated. As a result, when the mold temperature was set to 20 ° C., the molding limit was reached at an early stage regardless of whether the molding start temperature was 800 ° C. or 750 ° C., but the mold temperature was 600 ° C. When it was set to, good moldability was achieved regardless of whether the molding start temperature was 800 ° C. or 750 ° C., and deep-draw molding was achieved up to the bottom dead center of molding.

Based on these results, further investigations were made. As a result, after the thin steel plate is heated to a temperature equal to or higher than the Ac ₃ transformation point, the temperature difference in the thin steel plate is 200 at the initial stage of forming, from the start of forming to 1/3 of the forming height. It has been found that if the temperature is within the range of ° C., good moldability can be ensured until the end.

  The reason why such a phenomenon occurs can be considered as follows. That is, if there is a temperature distribution in which the temperature difference in the thin steel plate exceeds 200 ° C. at the initial stage of forming, which reaches 1/3 of the forming height after starting forming, as shown in FIG. It is considered that local deformation (in FIG. 2, the local deformation portion is indicated by A) is likely to occur during molding, and the moldability deteriorates. On the other hand, when the temperature difference in the thin steel plate is within 200 ° C. at the initial stage of forming, the above-described local deformation is unlikely to occur, and good formability is considered to be exhibited. The above temperature difference is preferably within 150 ° C. (more preferably within 100 ° C.). However, if the temperature difference is made too small, control becomes difficult and workability is lowered.

Various means such as the following (1) to (4) can be adopted as means for setting the temperature difference in the steel sheet to 200 ° C. or less at the initial stage of forming after starting forming.
(1) Control the forming start temperature and the mold temperature to reduce the temperature difference in the steel sheet [for example, lower the forming start temperature or increase the mold temperature (or use in combination)].
(2) The forming speed is controlled (for example, the forming speed is increased so that the heat conduction time between the steel plate and the mold is shortened).
(3) Decrease the heat transfer coefficient between the blank and the mold (for example, use ceramics as the mold material to make it difficult for the heat of the steel plate to be transmitted to the mold).
(4) Molding is performed while cooling the parts other than the wrinkle presser (for example, molding is performed while feeding air and cooling gas into the mold).

  In the present invention, if the requirement that “the temperature difference in the steel sheet is within 200 ° C.” is satisfied at the early stage of forming, the above-described effect is exhibited, and the forming start temperature is limited. However, the molding may be started at a temperature higher than the martensite transformation start temperature Ms, or the molding may be started at a temperature equal to or lower than the martensite transformation start temperature Ms. In particular, when forming is started at a temperature equal to or lower than the martensite transformation start temperature Ms, the temperature range is such that the surface oxidation of the steel sheet is difficult to occur, and thus there is an advantage that the formation of scale on the steel sheet surface can be avoided.

The temperature at the Ac ₃ transformation point of the steel sheet shown in Table 1 means the temperature at which transformation to austenite is completed when the steel sheet is heated, and is obtained by the following equation (1). Further, the above-described martensitic transformation start temperature Ms is a value obtained by the following formula (2) (for example, “Heat Treatment” 41 (3), 164 to 169, 2001, Kunitake Tatsuro “Steel Ac ₁ , Ac ₃ And prediction of Ms transformation point by empirical formula ").
Ac ₃ transformation point (° C.) = − 230.5 × [C] + 31.6 × [Si] −20.4 × [Mn] −39.8 × [Cu] −18.1 × [Ni] -14. 8 × [Cr] + 16.8 × [Mo] +912 (1)
Ms (° C.) = 560.5− {407.3 × [C] + 7.3 × [Si] + 37.8 × [Mn] + 20.5 × [Cu] + 19.5 × [Ni] +19.8 [Cr ] + 4.5 × [Mo]} (2)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass of C, Si, Mn, Cu, Ni, Cr and Mo, respectively). %).

  In carrying out press forming using the apparatus of the present invention, the above-mentioned object can be achieved by appropriately controlling the temperature distribution in the steel sheet at the initial stage of forming. This is remarkably exhibited when cylindrical deep drawing is performed using a mold having a presser (molding in which temperature distribution tends to occur). However, the apparatus of the present invention includes the case of deep drawing of a rectangular tube and the case of performing ordinary press molding (for example, stretch molding). The effect is achieved. Moreover, in this invention apparatus, it is only necessary to control steel plate temperature distribution, and it is not necessary to make a metal mold | die structure complicated.

  Hereinafter, the effects of the present invention will be described more specifically by way of examples. However, the following examples are not intended to limit the present invention, and any design changes in accordance with the gist of the preceding and following descriptions are technical aspects of the present invention. It is included in the range.

Steel having the chemical composition shown in Table 1 was rolled to a thickness of 1.0 mm or 1.4 mm by ordinary means. From this, a circular blank having a diameter (blank diameter): 100 mm was punched and used for the experiment (accordingly, Ac ₃ transformation point of this blank: 830 ° C., martensite transformation start temperature Ms: 405 ° C.).

  Using the circular blank, a die having a circular punch head shape (diameter: 49.75 mm) (cylindrical die and cylindrical punch) is used (see FIG. 1), and cylindrical deep drawing is performed according to the method of the present invention. It was. At this time, the blank was heated using an electric furnace (without atmospheric control), and the heating temperature was set to 900 ° C.

  The molding experiment was performed using the mold shown in FIG. 1 and installed in a crank press. At this time, the time from when the mold contacts the blank until it stops at the bottom dead center of forming (forming time) was adjusted in the range of 0.1 to 0.7 seconds, and the temperature difference in the steel sheet was adjusted. . Other press molding conditions are as follows.

(Other press molding conditions)
Wrinkle holding force: 3 tons Die shoulder radius rd: 5mm
Punch shoulder radius rp: 5mm
Punch-die clearance CL: 1.32 / 2 + [1.0 or 1.4 (steel plate thickness)] mm
Molding height: 37mm
Lubricant: An oxidized Ca-based paste lubricant was used and applied to the mold.

The results are shown in Table 2 below. In Table 2, the mark “◯” indicates that good moldability was achieved without breaking or cracking, and that deep drawing was achieved to the bottom dead center of the molding (the state shown in FIG. 1). The symbol “x” means that breakage or cracking occurred during molding (for example, the state shown in FIG. 2). For those that could be formed, the temperature difference in the steel sheet at the initial stage of forming was measured with a radiation thermometer with a laser, and the temperature distribution during forming was calculated by numerical simulation. Turned out to be controlled. On the other hand, in the case where fractures or cracks occurred during forming, the temperature difference of the steel sheet at the initial stage of forming exceeded 200 ° C.

  From this result, it is apparent that good moldability can be ensured by adjusting the molding time so that no temperature distribution occurs.

1 Punch 2 Die 3 Blank holder 4 Blank (steel plate)

Claims (4)

A heating furnace for heating the thin steel sheet to a temperature above the Ac ₃ transformation point, a cooling means for cooling the thin steel sheet to a temperature of at least 600 ° C. at a speed equal to or higher than a critical cooling rate, and press forming the cooled thin steel sheet And a press forming machine provided with means for press forming so that the temperature difference in the thin steel sheet is within 200 ° C. when the forming height reaches 1/3. An apparatus for manufacturing press-formed products.   The manufacturing apparatus of Claim 1 comprised so that shaping | molding may be started at the temperature higher than the martensitic transformation start temperature Ms.   The manufacturing apparatus of Claim 1 comprised so that shaping | molding may be started at the temperature below martensitic transformation start temperature Ms.   The manufacturing apparatus according to any one of claims 1 to 3, wherein the forming is performed by using a wrinkle presser.

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