JP7372787B2 - Hot press forming method and hot press forming product - Google Patents

Description

The present invention relates to a hot press forming method and a hot press forming product.

BACKGROUND ART In the automobile field and the like, hot press-formed products (also referred to as "hot-pressed products" or "hot-stamped products") are used from the viewpoint of improving strength and reducing weight. A hot press-formed product is one in which a plate-shaped blank material (steel plate) made of steel is hot press-formed. For example, a steel plate is heated until the metallographic structure becomes austenite, then press-formed in a mold, and It is hardened by cooling the mold together in the pressed state.

As mentioned above, hot press-formed products have high strength due to being quenched, but for example, parts used in automobiles etc. are subject to post-processing such as piercing, trimming, and welding. Therefore, it is desirable that the strength of the parts to be subjected to these post-processings is not too high. For this reason, for example, Patent Documents 1 and 2 describe methods for forming hot press-formed products having different strengths depending on their parts.

Japanese Patent Application Publication No. 2018-79484 International Publication No. 2013/137308

Under these circumstances, the present invention provides a hot press forming method for obtaining a hot press molded product having different characteristics depending on the part by a method different from the conventional method, and a hot press forming product having different characteristics from the conventional method. The purpose is to

The present inventors have found that the above problem can be solved by the following configuration.

<1>
a step (1) of heating a steel plate to make the entire steel plate austenite;
a step (2) of press-cooling the entire steel plate after the step (1) to make the entire steel plate martensite;
A step (3) of heating the steel plate after the step (2) to make a first region, which is a part of the steel plate, austenite and a second region, which is another part of the steel plate, into tempered martensite. )and,
a step (4) of pressing and cooling the entire steel plate after the step (3) to make the first region martensite and forming the steel plate;
A hot press forming method comprising:
<2>
The hot press forming method according to <1>, wherein the heating in the step (1) and the heating in the step (3) each take a heating time of 30 seconds or less from the start of heating to the completion of heating.
<3>
The hot press forming method according to <1> or <2>, wherein the heating in the step (1) and the heating in the step (3) are resistance heating.
<4>
The hot press forming method according to any one of <1> to <3>, wherein the heating in the step (3) is performed by changing the maximum temperature reached by heating for each part of the steel plate.
<5>
According to any one of <1> to <4>, the maximum temperature of the second region reached by heating in the step (3) is 400 ° C. or higher and lower than the austenite transformation start temperature (A ₁ point). Hot press forming method.
<6>
A hot press-formed product having a first region made of martensite and a second region made of tempered martensite, the martensite in the first region having a grain size number of 10 or more.

According to the present invention, it is possible to provide a hot press molding method for obtaining a hot press molded product having different characteristics depending on the part by a method different from the conventional method, and a hot press molded product having different characteristics from the conventional method. .

(A) to (E) are schematic diagrams of an example of a hot press forming method for explaining an embodiment of the present invention. FIG. 3 is a schematic diagram for explaining a bending test based on the VDA standard. FIG. 3 is a schematic diagram for explaining a VDA bending angle. (A) to (E) are schematic diagrams of an example of a hot press forming method for explaining an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of a hot press-formed product manufactured by the hot press-forming method of the present invention.

[Hot press forming method]
A hot press forming method according to one embodiment of the present invention includes:
a step (1) of heating a steel plate to make the entire steel plate austenite;
a step (2) of press-cooling the entire steel plate after the step (1) to make the entire steel plate martensite;
A step (3) of heating the steel plate after the step (2) to make a first region, which is a part of the steel plate, austenite and a second region, which is another part of the steel plate, into tempered martensite. )and,
a step (4) of pressing and cooling the entire steel plate after the step (3) to make the first region martensite and forming the steel plate;
has.

FIG. 1 is a schematic diagram of an example of a hot press forming method for explaining an embodiment of the present invention.

The steel plate 10 in FIG. 1A shows the steel plate in a state before performing step (1). The metallographic structure of the steel plate (steel plate 10 in FIG. 1A) before performing step (1) usually includes ferrite (F) and cementite (θ). The metal structure of the steel plate (steel plate 10 in FIG. 1A) before step (1) is, for example, one consisting of ferrite (F) and cementite (θ), or one consisting of ferrite (F) and pearlite (P). ), and those consisting of ferrite (F), cementite (θ), and pearlite (P).
Note that the composition of the steel plate is not particularly limited as long as it can be hardened.

<Step (1)>
Step (1) is a step of heating the steel plate to turn the entire steel plate into austenite.
FIG. 1B shows a state in which step (1) has been performed and the metal structure of the entire steel plate 11 is austenite (γ).

The maximum temperature of the steel plate reached by heating in step (1) is preferably at least _A3 point, which is the temperature at which the transformation from ferrite to austenite is completed, and is preferably 850 to 950°C, for example.

In the heating in step (1), it is preferable that the heating time from the start of heating to the completion of heating is 30 seconds or less. By heating in step (1) in such a rapid and short-time manner, productivity can be improved.

The heating method in step (1) is not particularly limited. Examples of the heating method include furnace heating, resistance heating (e.g., induction heating, electrical heating), etc., but resistance heating is preferable because rapid short-term heating is easy, and electrical heating is preferred. It is more preferable that there be. The specific heating method is not particularly limited, and any known method can be used.

Note that the shape of the steel plate is not particularly limited as long as it is plate-like. When conducting electrical heating as the heating method, the steel plate is preferably a flat plate. In addition, in the case of a steel plate whose cross-sectional area monotonically increases or decreases along the longitudinal direction, such as when the shape seen from the thickness direction of the steel plate is trapezoidal, energization is applied while moving at least one electrode in the longitudinal direction. By heating, the steel plate can be heated uniformly.
The length, width, and thickness (plate thickness) of the steel plate are not particularly limited, and can be appropriately selected depending on the specifications of the hot press-formed product.

<Step (2)>
Step (2) is a step of press-cooling the entire steel plate after step (1) to turn the entire steel plate into martensite.

In step (2), the steel plate that has undergone step (1) and has been heated so that the entire metal structure is austenite is press-cooled and quenched to transform the entire metal structure of the steel plate into martensite.
Press cooling is an operation in which press molding is performed using a press mold and cooling is performed within the press mold. The specific method of press cooling is not particularly limited, and any known method can be used. Note that the cooling temperature and cooling rate in press cooling are not particularly limited as long as they are within a range that allows quenching.

(C) of FIG. 1 shows an example of the state immediately after performing press cooling in step (2). The steel plate 11 after step (1) is press-formed using a press mold 20 and cooled to obtain a steel plate 12 whose metallographic structure is martensite (M).

<Step (3)>
Step (3) is to heat the steel plate after step (2) to make a first region, which is a part of the steel plate, austenite, and to make a second region, which is another part of the steel plate, tempered marten. This is the process of creating a website.

In step (3), the steel plate whose entire metal structure is martensite after step (2) is heated again to transform a part of the steel plate into austenite, and to transform the other part of the steel plate into tempered martensite. Make it a site.
Moreover, in step (3), a region (third region) containing austenite and tempered martensite may be formed between the first region (austenite) and the second region (tempered martensite).

In FIG. 1D, step (3) is performed, and the metal structure of the first region (1A), which is a part of the steel plate 13, becomes austenite (γ), and the second region (2A, which is another part of the steel plate) becomes austenite (γ). ) shows a state in which the metal structure is tempered martensite (M _T ).

The heating in step (3) is preferably performed by changing the maximum temperature reached by heating for each part of the steel plate.
The maximum temperature in the first region reached by heating in step (3) is preferably at least _A3 point, and is preferably 850 to 950°C, for example.
The maximum temperature in the second region reached by heating in step (3) is not particularly limited as long as it can be tempered, but it is preferably 400° C. or higher and lower than the austenite transformation start temperature (A ₁ point).
By adjusting the maximum temperature of the second region reached by heating in step (3), the hardness of the second region can be adjusted in the resulting hot press-formed product.
Further, the width of the third region can be adjusted by, for example, adjusting the temperature difference between the maximum temperature of the first region and the maximum temperature of the second region reached by heating in step (3), or the heating time. can. For example, the width of the third region can be narrowed by reducing the temperature difference or shortening the heating time.

In the heating in step (3), it is preferable that the heating time from the start of heating to the completion of heating is 30 seconds or less. By performing the heating in step (3) rapidly and in a short time in this manner, productivity can be improved.

The heating method in step (3) is not particularly limited. Examples of the heating method include furnace heating, resistance heating (e.g., induction heating, electrical heating), etc., but resistance heating is preferable because rapid short-term heating is easy, and electrical heating is preferred. It is more preferable that there be. The specific heating method is not particularly limited, and any known method can be used.

In step (3), the specific means for changing the maximum temperature reached by heating in the first region and the second region is not particularly limited. In particular, it is preferable that the heating is carried out by electrically heating the entire steel plate and cooling the second region. The method of cooling the second region in this case is not particularly limited, and is preferably carried out by, for example, spraying a cooling medium (cooling gas, etc.) or bringing a mold into contact with it. The specific methods of spraying the cooling medium and cooling the mold by contacting the mold are not particularly limited, and known methods can be used.

<Step (4)>
Step (4) is a step of press-cooling the entire steel plate after step (3) to make the first region martensite and forming the steel plate.

After step (3), the first region whose metal structure is austenite is quenched in step (4) and transformed into martensite.
The second region whose metal structure is tempered martensite after step (3) remains tempered martensite even after step (4).
In addition, if a region (third region) containing austenite and tempered martensite is formed between the first region (austenite) and the second region (tempered martensite) through step (3), this After step (4), the third region becomes a region containing martensite and tempered martensite.

(E) of FIG. 1 shows an example of the state immediately after performing press cooling in step (4). The steel plate after step (3) is press-formed by a press die 21 and cooled to become a hot press-formed product 14. In the hot press-formed product 14, the metal structure of the first region (1B) is martensite (M), and the metal structure of the second region (2B) is tempered martensite (M _T ).
The first region (1B) of the hot press-formed product 14 is a region with high strength (hard part) made of martensite, and the second region (2B) is a part (hard part) with lower strength than the hard part made of tempered martensite. software department). The soft part is easy to perform post-processing such as piercing, trimming, and welding.

In the press cooling step (4), the steel plate is press-formed into a desired shape to obtain a hot press-formed product. The specific method of press cooling is not particularly limited, and any known method can be used. Note that the cooling temperature and cooling rate in press cooling are not particularly limited as long as the first region can be hardened.
Moreover, the shape in press molding is not particularly limited. For example, it may be a flat plate as shown in Fig. 1 (E), or it may have a hat-shaped cross section as shown in Fig. 5, or any other shape depending on the use and specifications of the hot press-formed product. It can have any desired shape.

As mentioned above, in the hot press forming method of the present invention, the heating time in step (1) and the heating in step (3) is preferably within 30 seconds from the start of heating to the completion of heating. preferable. In this way, by performing heating in step (1) and heating in step (3) in a rapid and short time, it is possible to not only improve productivity but also improve the properties of the resulting hot press-formed product. can. That is, the first region of the hot press-formed product obtained by performing rapid short-time heating as the heating in step (1) and the heating in step (3) is In steps 3) and 4), the process of rapid short-time heating and quenching is performed twice, so the metal structure has a fine grain size (grain size number measured based on JIS G0551 (2013)). (10 or more) martensite. It is known that strength improves when the crystal grain size of the metal structure becomes finer.

[Hot press molded product]
A hot press-formed product according to one aspect of the present invention has a first region made of martensite and a second region made of tempered martensite, and the martensite in the first region has a grain size number of 10 or more. be.

A hot press-formed product 14 in FIG. 1 schematically shows an example of a hot press-formed product of the present invention. The hot press-formed product 14 has a first region (1B) made of martensite (M) and a second region (2B) made of tempered martensite ( _MT ), and the martensite in the first region The grain size number is 10 or more.

The hot press-formed product of the present invention is preferably heated in step (1) and step (3) in the above-described hot press-forming method of the present invention, respectively, from the start of heating to the completion of heating. It can be obtained by heating within 30 seconds.

In the hot press-formed product of the present invention, the number of portions corresponding to the first region and the second region may be only one, or each may be multiple.
The hot press-formed product of the present invention may or may not have portions other than the first region and the second region. The portion other than the first region and the second region includes a third region containing martensite and tempered martensite. When the hot press-formed product of the present invention has a third region, it is preferable to have the third region between the first region and the second region. That is, the hot press-formed product of the present invention may consist of only the first region and the second region, or may consist of the first region, the second region, and the third region.
In the hot press-formed product of the present invention, the first region is a hard part, and the second region is a soft part.

Further, in the hot press-formed product of the present invention, the grain size number of the martensite in the first region measured based on JIS G0551 (2013) is 10 or more, preferably 11 or more, and more preferably 11. .5 or more.

The hot press-formed product of the present invention is not limited in form or use, but includes, for example, vehicle bodies, bumpers, oil pans, inner panels, pillars (e.g. A-pillar, B-pillar, C-pillar, D-pillar, etc.), It can be used as a foil house, etc.

EXAMPLES The present invention will be described in more detail below based on Examples, but the scope of the present invention should not be construed as being limited by the Examples shown below.

[Example 1]
<Step (1)>
A 1500 MPa class steel plate for hot stamping was suspended between the left and right electrodes of a direct current heating device, the steel plate was sandwiched between the upper and lower electrodes, and electricity was applied between the left and right electrodes. The entire steel plate was heated from room temperature to 900° C. for 20 seconds to transform the metal structure from ferrite and pearlite to austenite.

<Step (2)>
When the temperature of the steel plate reached 900°C, the electricity supply was stopped and press cooling was immediately performed. Press cooling is performed using a forming mold that is equipped with a channel inside that guides cooling water to rapidly cool the steel plate. While forming the steel plate into a flat plate, the steel plate is rapidly cooled and quenched to change the metallographic structure of the entire steel plate. transformed into martensite. The press cooling time (press mold bottom dead center holding time) was 5 to 10 seconds.

<Step (3)>
The flat steel plate obtained in step (2) and entirely made of martensite was heated again using the direct current heating device used in step (1). Here, the first region, which is a part of the steel plate, was heated from room temperature to 900° C. in 20 seconds. Further, the second region, which is another part of the steel plate, was electrically heated by controlling the maximum temperature reached by heating the second region so that the temperature reached 700° C. in 20 seconds from room temperature. By this electrical heating, the first region becomes austenite and the second region becomes tempered martensite.

<Step (4)>
In step (3), when the temperature of the first region of the steel plate reached 900° C. and the temperature of the second region reached 700° C., the electricity supply was stopped and press cooling was immediately performed. Press cooling was performed in the same manner as in step (2). The steel plate was rapidly cooled while being formed into a flat plate by press cooling to obtain a hot press-formed product. The press cooling time (press mold bottom dead center holding time) was 5 to 10 seconds. When the obtained hot press-formed product was examined, the metal structure in the first region was martensite, and the metal structure in the second region was tempered martensite.

[Evaluation of mechanical properties]
The Vickers hardness was measured for each of the first region (martensite) and the second region (tempered martensite) of the obtained hot press-formed product. Using a Vickers hardness meter, measurements were made at 5 points at 300 gf (load 300 g, HV 0.3) in accordance with the regulations of JIS Z2244 (2009), and the average value was determined.
As a result, the first region was 523 HV, and the second region was 273 HV. From these results, it was found that in Example 1, a hot press-formed product was obtained that had a hard part with excellent strength and a soft part that was easy to post-process.

In addition, it was confirmed by conducting a separate test that the hardness of the second region, which is the soft part, can be adjusted by changing the maximum temperature of the second region reached by heating in step (3). As a result, it was found that the hardness of the second region can be adjusted within a range up to a lower limit of 270 HV (approximately 850 MPa). From this result, it was found that the hot press-formed product of the present invention allows the characteristics of the soft portion to be adjusted according to various uses and specifications.

[Measurement of grain size]
The crystal grain size of martensite in the first region of the obtained hot press-formed product was measured based on JIS G0551 (2013). When measurements were taken at five points and the average value was calculated, the crystal grain size number was 11.9 (crystal grain size was about 6.5 μm).
For reference, the grain size of martensite in the steel plate after step (2) and before step (3) was similarly measured at 5 points and the average value was calculated, and the grain size number was 11.1 (crystal grain size is approximately 8.5 μm).
From this result, it was found that in Example 1, the crystal grain size of martensite in the first region was refined by performing the quenching after rapid short-time heating twice.

[Example 2]
Next, in order to evaluate the toughness of the soft part of the hot press-formed product obtained by the hot press-forming method of the present invention, the bending strength was measured based on the VDA standard (VDA238-100) specified by the German Automobile Manufacturers Association. A test (bending test) was conducted.
The test piece used in the bending strength test was created as follows.

<Creation of test piece>
A plate-shaped test piece of 60 mm x 60 mm was prepared in the same manner as in Example 1 except that the first region was not provided in step (3) of Example 1.
This test piece is entirely made of tempered martensite, and corresponds to the soft part of a hot press-formed product obtained by the hot press-forming method of the present invention.

[Measurement of bending strength]
As shown in FIG. 2, a 60 mm x 60 mm plate-shaped test piece 32 is placed on two support rolls 31 with a diameter of 30 mm, and a punch 33 is pushed in at a speed of 20 mm/min. The radius of curvature of the tip of the punch is 0.4 mm. The interval between the rolls was set to the thickness of the plate-shaped test piece x 2 + 0.5 mm. The bending angle (VDA bending angle) when a crack appeared in the test piece (at maximum load) was determined. The larger the bending angle at maximum load, the higher the toughness at the time of crushing. As shown in FIG. 3, the bending angle is (180° - α) x 1/ where α is the angle formed by the bent test piece 34 after the bending test (α is 0° to 180°). This is the angle found in 2.

The test was conducted three times and the average value was calculated. As a result, the bending angle of the test piece of Example 2 was 60°.

[Comparative example 1]
For comparison, hot press-formed products were produced and evaluated using a method other than the hot press-forming method of the present invention, as shown below.

<Creation of hot press molded product of Comparative Example 1>
A 1500 MPa class steel plate for hot stamping was used.
The steel plate was electrically heated using a direct electrical heating device. At this time, a part of the steel plate was heated to 900°C to transform into austenite, and the maximum temperature reached by heating the other part was controlled to a temperature below point _A1 to maintain ferrite. . Thereafter, the austenite was transformed into martensite by press cooling and quenching. In this way, a hot press molded product of Comparative Example 1 was obtained. When we examined the hot press-formed product of Comparative Example 1, we found that part of it was martensite (this part is also called the "R(M) part"), and part of it was ferrite and pearlite (this part was also called the "R(M) part"). There is a part where ferrite and martensite are mixed (this part is also called the "R(F+M) part") between the R(M) part and the R(F+P) part. was.

The Vickers hardness of the R(F+M) portion of the hot press-formed product of Comparative Example 1 was determined. Using a Vickers hardness tester, measurements were made at 5 points at 300 gf (load 300 g, HV 0.3) in accordance with the regulations of JIS Z2244 (2009 edition), and the average value was determined. The result was 294HV.

[Comparative example 2]
A plate-shaped test piece corresponding to the R (F+M) portion of Comparative Example 1 was prepared, and a bending test was performed three times in the same manner as in Example 2, and the average value of the bending angle was determined. The bending angle of the test piece of Comparative Example 2 was 27°.

A comparison between Example 2 and Comparative Example 2 revealed that the soft portion of the hot press-formed product obtained by the hot press-forming method of the present invention has excellent toughness.

[Example 3]
<Step (1)>
A 1500 MPa class steel plate for hot stamping (length 1200 mm, width 500 mm, thickness 1 mm) was suspended between the left and right electrodes of a direct current heating device, the steel plate was sandwiched between the upper and lower electrodes, and electricity was applied between the left and right electrodes. The entire steel plate was heated from room temperature to 900° C. for 20 seconds to transform the metal structure from ferrite and cementite to austenite.
The steel plate 40 in FIG. 4A shows the state before step (1) is performed. The metal structure of the steel plate 40 in FIG. 4A consists of ferrite (F) and cementite (θ).
FIG. 4B shows a state in which step (1) has been performed and the metal structure of the entire steel plate 41 is austenite (γ).

<Step (2)>
When the temperature of the steel plate reached 900°C, the electricity supply was stopped and press cooling was immediately performed. Press cooling is performed using a forming mold that is equipped with a channel inside that guides cooling water to rapidly cool the steel plate. While forming the steel plate into a flat plate, the steel plate is rapidly cooled and quenched to change the metallographic structure of the entire steel plate. transformed into martensite. The press cooling time (press mold bottom dead center holding time) was 15 seconds.
(C) of FIG. 4 shows the state of the steel plate 42 after performing press cooling in step (2). The metal structure of the entire steel plate 42 is martensite (M).

<Step (3)>
The flat steel plate obtained in step (2) and entirely made of martensite was heated again using the direct current heating device used in step (1). Here, the first region, which is a part of the steel plate, was heated from room temperature to 900° C. in 20 seconds. Further, the second region, which is another part of the steel plate, was electrically heated by controlling the maximum temperature reached by heating the second region so that the temperature reached 600° C. in 20 seconds from room temperature. By this electrical heating, the first region becomes austenite, the second region becomes tempered martensite, and the third region between the first region and the second region becomes a region containing austenite and tempered martensite.
Note that the first region, the third region, and the second region were provided so that the metal structure changed in the length direction of the steel plate. The length of the first region was 800 mm, the length of the third region was 20 mm, and the length of the second region was 380 mm.
(D) in FIG. 4 shows that after step (3) is performed, the metal structure of the first region (1C) which is a part of the steel plate 43 becomes austenite (γ), and the second region (1C) which is another part of the steel plate 43 becomes austenite (γ). The metal structure of 2C) is tempered martensite ( _MT ), and the metal structure of the third region (3C) existing between the first region and the second region is austenite (γ) and tempered martensite ( _MT ). It shows a state consisting of.

<Step (4)>
In step (3), when the temperature of the first region of the steel plate reached 900° C. and the temperature of the second region reached 600° C., the electricity supply was stopped and press cooling was immediately performed. Press cooling is performed using a forming mold that is equipped with a water channel inside that guides cooling water to rapidly cool the steel plate, and the steel plate is rapidly cooled while being formed into a hat-shaped cross section in the width direction. It was quenched to obtain a hot press-formed product. The press cooling time (press mold bottom dead center holding time) was 15 seconds. When the obtained hot press-formed product was examined, the metal structure of the first region was martensite, the metal structure of the second region was tempered martensite, and the metal structure of the third region was martensite and tempered martensite. It was a site.
(E) of FIG. 4 shows the state of the hot press-formed product 44 manufactured by performing the press cooling in step (4). The steel plate after step (3) is press-formed using a press die (not shown) and cooled to become a hot press-formed product 44. In the hot press-formed product 44, the metal structure of the first region (1D) is martensite (M), the metal structure of the second region (2D) is tempered martensite ( _MT ), and the metal structure of the third region (3D) is martensite (M). ) is composed of martensite (M) and tempered martensite ( _MT ). The hot press-formed product 44 has a hat-shaped cross section in the width direction. FIG. 5 shows a perspective view of the hot press-formed product 44.

[Evaluation of mechanical properties]
The Vickers hardness was measured for each of the first region (martensite), third region (martensite and tempered martensite), and second region (tempered martensite) of the obtained hot press-formed product. Using a Vickers hardness meter, measurements were made at 5 points at 300 gf (load 300 g, HV 0.3) in accordance with the regulations of JIS Z2244 (2009), and the average value was determined.
As a result, the first region was 450-500 HV (1500-1700 MPa), the third region was 270-450 HV (850-1500 MPa), and the second region was 250-270 HV (800-850 MPa). From this result, in Example 3, a hot press having a hard part (first region) with excellent strength, a soft part (second region) that is easy to post-process, and a third region with an intermediate strength It was found that a molded article was obtained.

[Measurement of grain size]
The crystal grain size of martensite in the first region of the obtained hot press-formed product was measured based on JIS G0551 (2013). When measurements were taken at five points and the average value was calculated, the grain size number was 11.8.
From this result, it was found that in Example 3, as in Example 1, the grain size of martensite in the first region was refined by performing the rapid short-time heating and then quenching twice.

10, 11, 12, 13, 40, 41, 42, 43 Steel plate 14, 44 Hot press-formed product 20, 21 Press mold 1A, 1B, 1C, 1D First area 2A, 2B, 2C, 2D Second area 3C , 3D Third region F Ferrite θ Cementite P Pearlite γ Austenite M Martensite _MT Tempered martensite 31 Support roll 32 Test piece 33 Punch 34 Test piece after bending test α Angle formed by test piece after bending test

Claims (4)

a step (1) of heating a steel plate to make the entire steel plate austenite;
a step (2) of press-cooling the entire steel plate after the step (1) to make the entire steel plate martensite;
A step (3) of heating the steel plate after the step (2) to make a first region, which is a part of the steel plate, austenite and a second region, which is another part of the steel plate, into tempered martensite. )and,
a step (4) of pressing and cooling the entire steel plate after the step (3) to make the first region martensite and forming the steel plate;
has
The maximum temperature of the second region reached by heating in the step (3) is 400°C or more and lower than the austenite transformation start temperature (A ₁ point),
Hot press forming method. The hot press forming method according to claim 1, wherein the heating in the step (1) and the heating in the step (3) each take a heating time of 30 seconds or less from the start of heating to the completion of heating. The hot press forming method according to claim 1 or 2, wherein the heating in the step (1) and the heating in the step (3) are resistance heating. The hot press forming method according to any one of claims 1 to 3, wherein the heating in the step (3) is performed by changing the maximum temperature reached by heating for each part of the steel plate.

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