WO2013089167A1 - Hot press formed product, manufacturing method for same, and press forming equipment - Google Patents

Abstract

　When using a press forming mold to manufacture a hot press formed product having a high-strength region and a low-strength in a single component, a hot press formed product exhibiting superior productivity and component ductility during manufacture and exhibiting superior corrosion resistance when necessary is manufactured by heating a steel sheet to a temperature equal to or above the Ac₃ transformation point, and then fast cooling the high-strength region before initiating press forming.

Description

Hot press-formed product, method for manufacturing the same, and press-forming equipment

TECHNICAL FIELD The present invention relates to a hot press-formed product that requires strength as used for a structural member of an automobile part, a method for manufacturing the same, and a press-forming facility for manufacturing the press-formed product, and is particularly preheated. The present invention relates to a hot press-formed product that obtains a predetermined strength by quenching at the same time as forming a steel sheet when it is formed into a predetermined shape, a manufacturing method of such a hot press-formed product, and a press forming facility therefor. is there.

As one of the measures to improve the fuel efficiency of automobiles that originated from global environmental problems, the weight of the car body has been reduced, and it is necessary to increase the strength of steel sheets used in automobiles as much as possible. However, when the strength of steel sheets is increased to reduce the weight of automobiles, the elongation EL and r value (Rankford value) decrease, and the press formability and shape freezeability deteriorate.

In order to solve such a problem, the steel plate (work material) is heated to a predetermined temperature (for example, the temperature at which it becomes an austenite phase) to reduce the strength (that is, to facilitate forming), and then to the steel plate. Hot press molding method that secures strength after molding by molding with a low temperature mold (for example, room temperature) compared to providing shape and quenching heat treatment (quenching) using the temperature difference between the two. Is used in parts manufacturing.

According to such a hot press molding method, since the molding is performed in a low strength state, the spring back is reduced (good shape freezing property), and a strength of 1500 MPa class is obtained as a tensile strength by rapid cooling. Such a hot press forming method is called by various names such as a hot forming method, a hot stamping method, a hot stamp method, a die quench method, etc. in addition to a hot press method (for example, Patent Document 1).

FIG. 1 is a schematic explanatory view showing a mold configuration for carrying out the above hot press molding (hereinafter may be represented by “hot stamp”). 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.

A hot press forming facility provided with a press forming machine having a mold configuration as described above is disclosed in Non-Patent Document 1, for example. In this facility, a heating furnace for heating and softening a thin steel plate, an apparatus for conveying the heated thin steel plate, a press forming machine for press-forming the thin steel plate, and trimming the formed product (laser) Etc., and a device for performing correction processing to obtain a final shape.

When hot stamping (for example, hot deep drawing) using such a mold, forming is started in a state where the steel plate (blank) 4 is heated and softened (direct method). That is, in a state where the steel plate 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3, the steel plate 4 is pushed into the hole of the die 2 (between 2 and 2 in FIG. 1) by the punch 1, and the outer diameter of the steel plate 4 is reduced. While shrinking, it is formed into a shape corresponding to the outer shape of the punch 1. Further, by cooling the punch 1 and the die 2 in parallel with the forming, heat is removed from the steel plate 4 to the mold (punch 1 and die 2), and the bottom dead center of the forming (the punch tip is located at the deepest part). In the state shown in FIG. 1), the material is further quenched by holding and cooling (die quench). 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. Such a molding method is also disclosed in Patent Document 2, for example.

In the above-described procedure, a method (direct method) is shown in which a hot press-formed product having a simple shape as shown in FIG. 1 is simultaneously formed and quenched in a mold from the stage of a steel plate. The hot pressing method applied in (1) is not limited to the case of applying to such a construction method, but can also be applied to the case of manufacturing a molded product having a relatively complicated shape.

That is, when manufacturing a molded product having a relatively complicated shape, it may be difficult to make the final shape of the product by one press molding. In such a case, a method of performing cold press forming in a pre-process of hot press forming can be adopted (this method is called “indirect method”). The indirect method is a method in which a portion that is difficult to be molded is preliminarily molded to an approximate shape by cold working, and the other portions are hot press molded. If this method is adopted, for example, when a part having three uneven portions (mountains) of a molded product is formed, the two parts are formed by cold press molding, and then the third part is formed. Will be hot-formed.

The conventional hot stamping method has a problem that the hardness is increased by quenching, so that drilling and trimming become difficult as the shear resistance increases (particularly when the direct method is adopted). Moreover, even if it can process, in addition to the influence of a tool breakage and tool wear, since a high residual stress arises in the cutting surface vicinity of a blank material, there exists a possibility of causing a delayed fracture. Therefore, for example, laser cutting is used when trimming, but application of laser cutting is expensive and causes high costs. For these reasons, there is a demand for a method that enables trimming by a mold (sometimes referred to as “die trimming”). Moreover, since rapid cooling (die quench) is performed in the mold, it is necessary to hold the mold at the bottom dead center for a certain period of time, and the low productivity also causes high costs.

A collision-resistant safety part in an automobile body structure needs to have both functions of an impact resistant part and an energy absorbing part. In order to produce such parts, the mainstream method has so far been laser-welded (Tailored Weld Blanks: TWB) of high strength high tensile strength and high strength high strength tensile strength steel and then cold-pressed. It was. On the other hand, in a molded product obtained by the conventional hot stamping method, the material ductility decreases as the strength increases, so that it becomes difficult to exhibit the function as an energy absorbing portion where ductility is required. Further, since the strength is increased by die quenching (die quench), material softening tends to occur in a heat affected zone (HAZ) formed during spot welding. When such a portion is subjected to an impact load, the breakage starting from the HAZ is also a cause of reducing the ductility of the component.

As a method for avoiding the above problems, a technique in which the strength of a part of the hot stamped product is intentionally reduced, that is, a technique in which different strength regions (high strength region and low strength region) are separately created in one part is also proposed. Such technology is called tailored tempering or soft flange. As such a technique, for example, in Patent Document 3, a mold contacting with a low-strength region is recessed to reduce the contact area, or a heating means is provided to partially increase the mold temperature, or the mold material A technique has been proposed in which a low-strength region is created by using a material having a low thermal conductivity and slowing the cooling rate.

However, in such a technique, since the low strength region is created by slowing down the cooling rate in the mold, the time for keeping the molded product in the mold (die quench time) becomes longer, As a result, the tact time is inevitably longer than that of hot stamping, and productivity is greatly reduced, resulting in higher costs. Moreover, since a heater is embedded in the mold or a recess is provided, the mold structure becomes complicated, which also causes high costs.

In the technique of Patent Document 4, in addition to changing the cooling rate in the cooling process as in the technique of Patent Document 3, a method of providing a temperature distribution in the heating process has been proposed. In order to obtain high strength by the hot stamping technique, it is necessary to heat so as to be in the temperature range of the austenite single phase (temperature range above the Ac ₃ transformation point) in the heating process. Utilizing this technique, the temperature of the high strength region is heated to the Ac ₃ transformation point or higher in the heating process, and the temperature of the low strength region is heated to a temperature lower than the Ac ₃ transformation point, and press molding and die quenching are performed. It is.

However, in this method, in order to receive a temperature distribution in the heating process, it is necessary to provide a shielding plate in the heating device, and much time and cost are required for remodeling the device. Moreover, when the design change of a part is implemented, it will be necessary to change a shielding board in connection with it, and will become complicated.

On the other hand, as a required characteristic for a molded product, good corrosion resistance is required particularly in the lower part of the vehicle body (underbody). For this reason, as a steel plate used as a material of the press-formed product, a surface-treated steel plate (plated steel plate) whose surface is plated is used to improve the corrosion resistance of the press-formed product. Currently, Al—Si-based plated steel sheets are widely used as hot stamped plated steel sheets used in the direct method.

As a material that exhibits sacrificial corrosion resistance, a surface-treated steel sheet (Zn—Fe-based plated steel sheet) in which a Zn—Fe-based plated layer containing a predetermined amount of Fe is formed on the substrate surface is used. However, when such a surface-treated steel sheet is used, in the indirect method, liquid metal embrittlement (Liquid) that causes delayed fracture occurs during press forming, in which molten zinc enters the grain boundaries of the steel sheet subjected to tensile stress. -There is another problem of occurrence of Metal Emblemment (LME), making it difficult to apply the molded product to the underbody.

JP 2002-102980 A JP 2007-275937 A JP 2003-328031 A JP 2007-75834 A

The present invention has been made in view of the above circumstances, and its purpose is to perform hot press molding, which is excellent in productivity at the time of production, part ductility, and if necessary, hot press molded article having excellent corrosion resistance, It is another object of the present invention to provide a useful method for producing such a hot press-formed product and an equipment for producing such a hot press-formed product.

The present invention method which was able to achieve the above object, a hot press molded product having a high strength region and low intensity regions within a single component, when prepared using a press molding die, the steel sheet Ac ₃ After heating to a temperature equal to or higher than the transformation point, prior to starting press molding, the high strength region is rapidly cooled.

As specific conditions in the method of the present invention, the temperature of the steel sheet before press forming is 600 ° C. or lower in the high strength region, the martensite transformation start temperature Ms or higher, and 650 ° C. or higher in the low strength region. The time from the start of rapid cooling to the start of press molding is preferably within 30 seconds.

In addition, the temperature of the molded product at the time of mold release after press molding is not more than the martensite transformation start temperature Ms + 100 ° C. in the high strength region, is not less than the martensite transformation end temperature Mf, and is in the low strength region (martensite transformation start temperature Ms + 100 ° C.) or higher.

In the method of the present invention, (a) the molded product may be allowed to cool after release, but (b) after molding, the high-strength region has an average cooling rate of 5 ° C./second or more and the martensite transformation end temperature. You may make it cool to the temperature below Mf.

When a surface-treated steel sheet having a Zn—Fe plating layer formed on the surface of the base steel sheet is used as the steel sheet used in the present invention, the forming start temperature is equal to or lower than the freezing point of the plating layer according to the Fe content in the plating layer. Preferably, the temperature is The steel plate used in the present invention is not limited to a Zn—Fe-based plated steel plate, and a non-plated steel plate or an Al—Si-based plated steel plate may be used.

The present invention also includes a hot press-molded product obtained by the manufacturing method as described above. Moreover, as a press molding facility for producing such a hot press-formed product, a heating furnace and a press molding machine are provided, and a high strength region of a heated steel plate is provided between the heating furnace and the press molding machine. It is useful to provide a cooling part for partially quenching the water.

According to the present invention, when a hot press molded product having a high strength region and a low strength region in a single part is produced using a press molding die, the steel plate is heated to a temperature equal to or higher than the Ac ₃ transformation point. Later, prior to the start of press molding, the high-strength region was rapidly cooled, so a hot press-formed product with excellent ductility in the low-strength region and excellent part ductility can be produced with high productivity. .

It is a schematic explanatory drawing which shows the metal mold | die structure for implementing hot press molding. It is a schematic diagram which shows the image of a heat pattern. It is a Fe-Zn binary system state diagram. It is a schematic explanatory drawing which shows the structural example of the conventional hot press molding equipment. It is a schematic explanatory drawing which shows an example of the press molding equipment of this invention. It is a schematic explanatory drawing which shows the structural example of the cooling part of the press molding equipment of this invention. It is a schematic explanatory drawing which shows the other structural example of the cooling part of the press molding equipment of this invention. It is a schematic explanatory drawing which shows the further another structural example of the cooling part of the press molding equipment of this invention.

The present inventors have studied from various angles in order to manufacture hot press-formed products having a high strength region and a low strength region in a single part with good productivity while ensuring the ductility of the part. As a result, when producing a hot press-formed product by press-forming a steel plate using a press-molding die, the steel plate is heated to a temperature equal to or higher than the Ac ₃ transformation point, and then, before starting press forming, high strength It has been found that if the region is rapidly cooled, a hot press molded product having excellent ductility in a low strength region and excellent component ductility can be produced with high productivity, and the present invention has been completed.

In the method of the present invention, it is necessary to heat the steel plate to a temperature equal to or higher than the Ac ₃ transformation point in order to exert the effect of the hot pressing method. When the heating temperature is less than the Ac ₃ transformation point, an appropriate amount of austenite cannot be obtained during heating, and good moldability cannot be ensured. The preferable lower limit of the heating temperature is not less than (Ac ₃ transformation point + 50 ° C.) of the steel sheet.

The upper limit of the heating temperature of the steel sheet is preferably up to about 1000 ° C. When this heating temperature is higher than 1000 ° C., the generation of oxide scale becomes significant (for example, 100 μm or more) during conveyance from the heating furnace to a press molding machine (see FIG. 5 below), and the thickness of the molded product (de There is a possibility that the thickness after scaling will be smaller than a predetermined value. Moreover, when using a plated steel plate, oxidation of a plating layer advances and plating quality may deteriorate. The upper limit with preferable heating temperature is 950 degrees C or less.

The image of the heat pattern of the method of the present invention is schematically shown in FIG. 2 (CCT curve: Continuous Cooling Transformation diagram). In the method of the present invention, a high-strength region (region desired to be a high-strength region: indicated by “region A” in the figure) is rapidly cooled before press molding, and a low-strength region (region desired to be a low-strength region: “ In the region B "), press molding is simultaneously performed at a high temperature. Then, the die quench time at the bottom dead center of molding is shortened as much as possible. By performing press molding under such conditions, in the region A rapidly cooled before press molding, a martensitic transformation (non-diffusion transformation) occurs without being affected by a nose that causes a diffusion transformation (boundary of diffusion type transformation). High strength. On the other hand, in the region B, the cooling rate after the mold release becomes slow due to the short time die quenching, so that the diffusion transformation occurs and the strength becomes low. In order to exert such an effect, the average cooling rate at the time of rapid cooling in the high strength region is preferably at least 27 ° C./second or more, more preferably 40 ° C./second or more.

At this time, as specific conditions, the time from the rapid cooling start (region A) before press forming to the start of press forming is within 30 seconds (more preferably within 20 seconds), and the temperature of the steel plate before press forming is In the high-strength region (region A), the temperature is preferably 600 ° C. or lower and the martensitic transformation start temperature Ms or higher. This temperature corresponds to the rapid cooling end temperature in the high strength region. On the other hand, in the low-strength region (region B), although the rapid cooling is not basically performed, the temperature of the steel sheet may decrease after heating and before press forming. In such a case, the low-strength region before press forming is assumed. The temperature is preferably 650 ° C. or higher. The die quench time (holding time at the molding bottom dead center) is preferably within 5 seconds (more preferably within 3 seconds).

The steel plate temperature (molded product temperature) at the time of mold release after press forming is (martensitic transformation start temperature Ms + 100 ° C.) or lower in the high strength region, martensitic transformation end temperature Mf or higher, and in the low strength region (martensitic transformation). It is preferable that the starting temperature is Ms + 100 ° C. or higher. By controlling in this way, higher strength can be achieved in the high strength region, and lower strength can be realized in the low strength region. The steel plate temperature (molded product temperature) at the time of mold release after press forming is more preferably not higher than the martensite transformation start temperature Ms in the high strength region and not lower than (martensite transformation start temperature Ms + 180 ° C.) in the low strength region. .

In the method of the present invention, the molded product may be allowed to cool after mold release. However, after molding, only the high-strength region has an average cooling rate of 5 ° C./second or more and a temperature equal to or lower than the martensite transformation end temperature Mf. You may make it cool (secondary cooling). As a result, the high-strength region is securely quenched, and higher strength of the region is achieved. In addition, the average cooling rate of the secondary cooling is more preferably 10 ° C./second or more, and further preferably 20 ° C./second or more.

According to the present invention, the high strength region is partially cooled before press molding, and the low strength region is basically formed without cooling before molding. By setting the area, die trimming can be performed, and it is not necessary to use a laser cutting machine, so that low cost can be realized. In addition, since die trimming in a low-strength region is possible, there is an advantage that the risk of wear and damage of the mold can be reduced, and this also leads to low cost. Furthermore, since the low-strength region is die-trimmed, high residual stress is not generated in the vicinity of the cut surface, and the fear of delayed fracture is eliminated. In addition, partial rapid cooling before press molding can shorten the die quench time, thereby improving productivity and reducing costs in this respect.

In the molded product obtained by the method of the present invention, the material ductility of the low strength region is improved, so that the moldability of the entire molded product is improved. In addition, if a molded product is manufactured assuming a low strength region in the flange portion where welding has been performed, material softening in the HAZ accompanying welding can be reduced, so the HAZ starting point is destroyed when subjected to an impact load. And the ductility of the molded product is improved.

Incidentally, when a Zn-based plated steel sheet is used as a press-molding material (blank), there is a problem of LME as described above. LME in hot stamping is thought to occur when molten zinc penetrates into a place where tensile stress is applied in a Zn-based plated steel sheet being formed. That is, if press molding is performed below the freezing point of the Zn-based plating layer, LME does not occur. Therefore, as shown in FIG. 2, only the region A including the bending portion where tensile stress is generated during press forming is rapidly cooled before press forming, and the solidification temperature of the Zn-based plating layer (the freezing point corresponding to the Fe content). : Indicated by “Fp” in FIG. 2), press forming so that the temperature is lower than that can increase the strength while avoiding LME. On the other hand, in other portions (low-strength region: region B shown in FIG. 2), tensile stress is not so much generated, so that LME is hardly generated. The low strength region is preferably formed at a temperature lower than the freezing point of the plating layer. However, since it is not significantly affected by press forming, it is not always necessary to form at a temperature lower than the freezing point. What is necessary is just to start shaping | molding in the temperature range from which intensity | strength is acquired.

FIG. 3 is a Zn—Fe binary system state diagram. The broken line portion in the figure indicates the boundary between the region including the liquid layer and the solid phase region (that is, corresponding to the freezing point: indicated by “Fp” in the drawing), and below this boundary is the solid phase region. Since the Zn-Fe-based plating layer has a different Fp temperature depending on the Fe content in the plating layer, the temperature of the galvanized layer before forming is equal to or less than Fp (corresponding to the freezing point) according to the Fe content. Cool down. It should be noted that not all of the Zn-Fe plating layer is immediately solidified when the surface-treated steel sheet is cooled to a temperature below the freezing point, but at least a part of the solid phase can be cooled by cooling to a temperature below the freezing point. By starting molding at the stage of precipitation, the above effects are exhibited.

In the method of the present invention, when a Zn—Fe-based plated steel sheet is used as a press-molding material (blank), the press-forming is performed at a temperature below the freezing point of the plated layer according to the Fe content in the Zn—Fe-based plated layer. Since this is done, there is basically no LME problem. Therefore, even in a press-molded product manufactured by hot stamping, the sacrificial anticorrosive property of the Zn—Fe-based plated steel sheet can be exhibited, and application to the underbody becomes possible.

The steel type of the steel plate used in the method of the present invention may be a normal chemical component composition as a high-strength steel plate. As such a steel plate, for example, a steel plate having the chemical composition shown in Table 1 below can be cited. The steel sheet has an Ac ₃ transformation point of 832 ° C., a martensite transformation start temperature Ms: 411 ° C., and a martensite transformation end temperature Mf: 261 ° C. The Ac ₃ transformation point, Ms, and Mf of the steel sheet are values obtained using the following formulas (1) to (3) (
For example, “Heat Treatment” 41 (3), 164 to 169, 2001 Tetsuro Kunitake “Predicting Ac ₁ , Ac ₃ and Ms transformation points of steel 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)
Mf (° C.) = Ms−150.0 (3)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr] and [Mo] are the contents (mass of C, Si, Mn, Cu, Ni, Cr and Mo, respectively). %). Moreover, when the element shown by each term of the said (1) Formula and (2) Formula is not contained, it calculates as the thing without the term.

In the present invention, the Fe content in the Zn—Fe-based plating layer formed on the surface of the steel sheet, if necessary, is not particularly limited, and may be plated if it is 5% by mass or more (more preferably 13% by mass or more). Although it functions as a layer, if the Fe content is excessive, corrosion resistance, coating film adhesion, weldability, etc. are likely to deteriorate, so it is preferably 80% by mass or less (more preferably 20% by mass). Less than).

The Zn—Fe-based plating layer may contain alloy elements other than Fe (eg, Al, Mn, Ni, Cr, Co, Mg, Sn, Pb, etc.) up to about 3.3 mass%. These elements have little influence on the freezing point at the content level. In addition to these components, the Zn—Fe plating layer has some inevitable factors such as Be, B, Si, P, Ti, V, W, Mo, Sb, Cd, Nb, Cu, and Sr. Impurities can also be included.

Incidentally, the conventional hot press line generally has a configuration (equipment configuration) as shown in FIG. 4 (schematic explanatory diagram). That is, as shown in FIG. 4, after the coiled steel sheet 10 is cut out by a cutting machine 11 (Blanking), it is heated in a heating furnace 12, and then conveyed to the press molding machine 13 to perform press forming. A press-formed product 14 is obtained.

In the present invention, after heating a steel plate to a predetermined temperature in a heating furnace, it is not directly conveyed to a press molding machine and molding is started, but molding is started after rapid cooling of a part (high strength region). Is. In carrying out such a method, for example, an equipment configuration as shown in FIGS. 5 to 8 (schematic explanatory diagrams) may be employed.

In this press forming equipment, a cooling unit 15 is provided inside the heating furnace 12 in association with the heating furnace 12, and a part of the steel sheet is rapidly moved until the steel sheet 10 is moved from the heating furnace 12 to the press forming machine 13. Cooling. The cooling unit 15 may be provided between the heating furnace 12 and the press molding machine 13 (see, for example, “cooling unit” or “cooling zone” in FIGS. 6 to 8). In the cooling performed by the cooling unit 15, for example, cooling can be performed by the following methods (1) to (4) (or in combination).

(1) A means for bringing into contact with a metal as a refrigerant (for example, a cooling means using a water-cooled roll) is provided for cooling.
(2) A gas cooling means is provided to cool the gas jet.
(3) Provide mist cooling means for cooling (for example, FIG. 8).
(4) A dry ice shot means (cooled by causing the granule dry ice to collide with the blank material) is cooled.

In the cooling using the cooling facility (cooling unit) of the present invention, it is also preferable to control the atmosphere simultaneously with the cooling. In particular, when using a non-plated steel sheet, the atmosphere can be controlled (for example, nitrogen or argon atmosphere) to prevent surface oxidation of the steel sheet. It is also possible to suppress surface oxidation by setting a relatively low temperature.

In the method of the present invention, the cooling conditions differ depending on different regions (high strength region and low strength region) of the steel sheet, but the cooling means (1) to (4) described above are used only for the high strength region. The cooling control corresponding to the steel plate region may be performed in the mold.

FIG. 6 is a schematic view showing a configuration example of the cooling unit, and shows a facility for cooling a heated thin steel plate by sandwiching it with a metal. The heated thin steel sheet is transported from the heating furnace to a flat mold for cooling (dedicated cooling mold), and the thin steel sheet is rapidly cooled to a predetermined temperature by pressing with this mold (cooling by holding metal). . After cooling, the steel sheet may be conveyed to a mold having a predetermined shape (depress-only mold) and press-molded. The shape of the cooling-only mold is preferably a flat surface on the steel plate contact surface side in order to cool the steel plate uniformly, but it is not necessarily flat for a temperature distribution or for some pre-forming. It is not necessary to have a step or curvature.

For example, as shown in FIG. 7, the steel plate is cooled to a predetermined temperature with a flat die (cooling-only die), and then sequentially pressed with a die having a predetermined shape. By molding, it is also possible to mold into a complicated shape (press die 1 and press die 2). Further, a step of imparting shape freezing property and a step of performing die trim piercing may be added.

Further, partial cooling before press forming is not limited to sandwiching metal, but may be a method of rapid cooling by spraying mist (or air) as shown in FIG. However, in mist and air, since the transition region between the cooling region and the non-cooling region becomes wide, in order to cool the target region (high-strength region) with high accuracy, it is preferable to perform cooling by sandwiching metal.

In FIGS. 6 and 7, a transfer press molding machine that continuously performs cooling means and mold molding is assumed, but the press molding machine used in the present invention is not limited to such a configuration, As long as a sufficient conveyance speed can be ensured, a configuration in which the press molding machine provided with each mold is separated may be used. From the viewpoint of shortening the press time, it is preferable to use a mechanical press (called a mechanical press) with a mechanical driving force for the pressure generating mechanism, but hydraulic pressure is used for the pressure generating mechanism. It may be a hydraulic press (for example, a hydraulic press).

This application claims the benefit of priority based on Japanese Patent Application No. 2011-272649 filed on December 13, 2011. The entire contents of Japanese Patent Application No. 2011-272649 filed on December 13, 2011 are incorporated herein by reference.

In producing a hot press-formed product having a high-strength region and a low-strength region in a single part using a press-molding die, the present invention, after heating the steel plate to a temperature equal to or higher than the Ac ₃ transformation point, Prior to the start of press molding, the high-strength region is rapidly cooled to obtain a hot press-molded product having excellent productivity and part ductility at the time of manufacture, and excellent corrosion resistance if necessary.

1 Punch 2 Die 3 Blank holder 4, 10 Steel plate (blank)
12 Heating furnace 13 Press molding machine 14 Press molded product 15 Cooling section

Claims (8)

1. In manufacturing a hot press-formed product having a high-strength region and a low-strength region in a single part using a press-molding die, the steel plate is heated to a temperature equal to or higher than the Ac ₃ transformation point, and then press forming is started. Prior to this, a method for producing a hot press-formed product, characterized in that the high-strength region is rapidly cooled.
2. The temperature of the steel plate before press forming is 600 ° C. or lower in the high strength region, the martensite transformation start temperature Ms or higher, and is 650 ° C. or higher in the low strength region, from the start of rapid cooling before press forming to the start of press forming. The method for producing a hot press-formed product according to claim 1, wherein the time is within 30 seconds.
3. The temperature of the press-molded product at the time of mold release after press molding is not more than (martensite transformation start temperature Ms + 100 ° C.) in the high strength region, is not less than the martensite transformation end temperature Mf, and is in the low strength region (martensite transformation start temperature Ms + 100). The method for producing a hot press-formed product according to claim 1 or 2.
4. The method for producing a hot press-formed product according to claim 3, wherein the molded product is allowed to cool after releasing.
5. 4. The method for producing a hot press-formed product according to claim 3, wherein after the molding, the high-strength region is cooled at an average cooling rate of 5 ° C./second or more to a temperature not higher than the martensite transformation end temperature Mf.
6. A surface-treated steel sheet having a Zn—Fe-based plating layer formed on the surface of the base steel sheet as the steel sheet, and the press forming start temperature is a temperature below the freezing point of the plating layer according to the Fe content in the plating layer. Item 3. A method for producing a hot press-formed product according to Item 1 or 2.
7. A hot press-formed product obtained by the production method according to claim 1 or 2.
8. An apparatus for producing a hot press-formed product according to claim 7, comprising a heating furnace and a press-forming machine, and a high-strength region of a heated steel plate between the heating furnace and the press-forming machine. A press forming facility characterized in that a cooling unit for partially quenching is provided.
   

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