WO2020080487A1

WO2020080487A1 - Hot stamping method and hot stamped product

Info

Publication number: WO2020080487A1
Application number: PCT/JP2019/040966
Authority: WO
Inventors: Yasuyuki Koyata; Hironori Ooyama; Maho Hosogi; Hirotaka Tanaka; Yoshitaka Misaka; Nobuyuki EHARA; Toshihiro Minagawa
Original assignee: Neturen Co., Ltd.; Arrk Corporation
Priority date: 2018-10-18
Filing date: 2019-10-17
Publication date: 2020-04-23

Abstract

A hot stamping method includes heating a steel plate such that a first region is transformed into austenite and a second region adjacent to the first region is transformed into ferrite and austenite, pressing and cooling the entire steel plate such that the first region is transformed into martensite and the second region is transformed into ferrite and martensite, reheating the steel plate such that the second region is transformed into ferrite and tempered martensite, a first part of the first region adjacent to the second region is transformed into tempered martensite, and a second part of the first region other than the first part is transformed into austenite, and pressing and recooling the entire steel plate after the reheating such that the second part of the first region is transformed into martensite and the steel plate is pressed into a desired shape.

Description

HOT STAMPING METHOD AND HOT STAMPED PRODUCT

The present invention relates to a hot stamping method and a hot stamped product.

In the field of automobiles and the like, a hot stamped product (also called "hot pressed parts") are used in view of high strength and low weight. A hot stamped product is obtained by hot stamping a blank made of steel (steel plate), for example, by quenching including pressing a steel plate in which a metallographic structure is heated to become austenite in a die, and cooling the steel plate together with the die in the pressed state.

As such, hot stamped products have high strength by being quenched. However, a part used in an automobile or the like may be subjected to piercing, trimming, and/or welding, and it is desirable that strength of a portion to be subjected to such post processing is not too high. A related art hot stamping method provides a hot stamped product having different strengths in different regions (see, e.g., JP2018-79484A and WO2013/137308A1).

Summary

Illustrative aspects of the present invention provides a hot stamping method for obtaining a hot stamped product having different characteristics in different regions by a method different from the related art, and a hot stamped product having different characteristics from the related art.

According to an illustrative aspect of the present invention, a hot stamping method includes heating a steel plate such that a first region of the steel plate is transformed into austenite and such that a second region of the steel plate other than the first region is transformed into ferrite and austenite, the second region being adjacent to the first region, pressing and cooling the entire steel plate after the heating such that the first region is transformed into martensite and such that the second region is transformed into ferrite and martensite, reheating the steel plate after the pressing and cooling such that the second region is transformed into ferrite and tempered martensite, such that a first part of the first region adjacent to the second region is transformed into tempered martensite, and such that a second part of the first region other than the first part is transformed into austenite, and pressing and recooling the entire steel plate after the reheating such that the second part of the first region is transformed into martensite and such that the steel plate is pressed into a desired shape.

According to another illustrative aspect of the present invention, a hot stamped product incudes a first region including a first part having a tempered martensite structure and a second part having a martensite structure, the second part being adjacent to the first part, and a second region having a ferrite and tempered martensite structure, the second region being adjacent to the first part of the first region but not adjacent to the second part of the first region. The grain size number of the martensite structure in the first part of the first region is equal to or greater than 10 in accordance with JIS G0551:2013.

Fig. 1A is a diagram illustrating an example of a steel plate. Fig. 1B is a diagram illustrating an example of a heated steel plate. Fig. 1C is a diagram illustrating an example of a press cooled steel plate. Fig. 1D is a diagram illustrating an example of a reheated steel plate. Fig. 1E is a diagram illustrating an example of a hot stamped product. Fig. 2A is a diagram illustrating another example of a steel plate. Fig. 2B is a diagram illustrating another example of a heated steel plate. Fig. 2C is a diagram illustrating another example of a press cooled steel plate. Fig. 2D is a diagram illustrating another example of a reheated steel plate. Fig. 2E is a diagram illustrating another example of a hot stamped product. Fig. 3 is a diagram illustrating a bending test according to the VDA standard. Fig. 4 is a diagram illustrating a VDA bending angle. Fig. 5A is a diagram illustrating yet another example of a steel plate. Fig. 5B is a diagram illustrating yet another example of a heated steel plate. Fig. 5C is a diagram illustrating yet another example of a press cooled steel plate. Fig. 5D is a diagram illustrating yet another example of a reheated steel plate. Fig. 5E is a diagram illustrating yet another example of a hot stamped product. Fig. 6 is a diagram illustrating an example of a hot stamped product produced by the hot stamping method illustrated in Figs. 5A to 5E.

Hot stamping Method

A hot stamping method according to one or more embodiments of the present invention include:
Step 1 - heating a steel plate such that a first region of the steel plate is transformed into austenite and such that a second region of the steel plate other than the first region is transformed into ferrite and austenite, the second region being adjacent to the first region;
Step 2 - pressing and cooling the entire steel plate after the heating such that the first region is transformed into martensite and such that the second region is transformed into ferrite and martensite;
Step 3 - reheating the steel plate after the pressing and cooling such that the second region is transformed into ferrite and tempered martensite, such that a first part of the first region adjacent to the second region is transformed into tempered martensite, and such that a second part of the first region other than the first part is transformed into austenite; and
Step 4 - pressing and recooling the entire steel plate after the reheating such that the second part of the first region is transformed into martensite and such that the steel plate is pressed into a desired shape.

Figs. 1A to 1E illustrate an example of a hot stamping method according to an embodiment of the present invention.

Fig. 1A illustrates a steel plate 10 in a state before Step 1 is performed. The metallographic structure of the steel plate 10 generally includes ferrite F and cementite θ. Examples of the metallographic structure of the steel plate 10 include a metallographic structure containing ferrite F and cementite θ, a metallographic structure containing ferrite F and pearlite P, and a metallographic structure containing ferrite F, cementite θ, and pearlite P. A composition of the steel plate is not particularly limited as long as the steel plate can be quenched.

Step 1

Step 1 is a step of heating the steel plate such that a first region of the steel plat is transformed into austenite and a second region of the steel plate adjacent to the first region is transformed into ferrite and austenite. Fig. 1B illustrates a state in which Step 1 is performed so that a metallographic structure of the first region 1A of the steel plate 11 is transformed into austenite γ, and a metallographic structure of the second region 2A of the steel plate 11 adjacent to the first region 1A is a region containing ferrite F and austenite γ.

The heating in Step 1 is preferably performed by changing a maximum temperature reached by heating each part of the steel plate. The maximum temperature to which the first region is heated in Step 1 is preferably equal to or higher than the A₃ point - the temperature at which transformation from ferrite to austenite is completed, for example, is preferably 850°C to 950°C.

The maximum temperature to which the second region is heated in Step 1 is preferably equal to or higher than the austenite transformation starting temperature (A₁ point) but lower than the austenite transformation finishing temperature (A₃ point), for example, is preferably 730°C to 800°C. By adjusting the maximum temperature of the second region which is reached by the heating in Step 1, the hardness of the second region in the hot stamped product can be adjusted.

For the heating in Step 1, heating time from the start of heating to the completion of heating is preferably within 30 seconds, i.e. not longer than 30 seconds. By setting the heating in Step 1 as rapid short time heating in this way, productivity can be improved.

A heating method in Step 1 is not particularly limited. Examples of the heating method include furnace heating and resistance heating (such as induction heating and direct resistance heating), but resistance heating is preferable, and direct resistance heating is more preferable due to a fact that the rapid short time heating is easy. A specific method of heating is not particularly limited, and a publicly known method can be used.

In Step 1, a specific method for changing the maximum temperature reached by heating in the first region and the second region is not particularly limited. In particular, the specific method is preferably performed by electrically heating the entire steel plate and cooling the second region. In this case, a method of cooling the second region is not particularly limited, and is preferably performed by, for example, spraying a cooling medium (for example, a cooling gas) or bringing a die into contact. Specific methods of the method of spraying the cooling medium and the method of bringing the die into contact is not particularly limited, and a publicly known method can be used.

A shape of the steel plate is not particularly limited as long as it is a plate shape. When direct resistance heating is performed as a heating method, the steel plate is preferably a flat plate. In addition, in a case where the steel plate has a trapezoidal shape viewed from a plate thickness direction and the like, in a case of the steel plate having a shape in which a cross-sectional area monotonously increases or decreases in a longitudinal direction, the steel plate can be uniformly heated by performing the direct resistance heating while moving at least one electrode in the longitudinal direction. Length, width, and thickness (plate thickness) of the steel plate are not particularly limited, and can be appropriately selected according to specifications of a hot stamped product and the like.

In Step 1, the second region of the steel plate is in a state in which ferrite and austenite are mixed, but may be a second region in which a composition ratio of the ferrite and austenite in the second region is constant in the longitudinal direction of the steel plate (also referred to as "constant structure ratio") or different. For example, the composition of the second region may be a composition in which a proportion of the austenite gradually increases from a part far from the first region toward a part close to the first region (also referred to as "inclined structure ratio"). In addition, the composition of the second region may have both a part having a constant structure ratio and a part having an inclined structure ratio. In Steps 2 to 4 after Step 1, austenite in the second region is martensite or tempered martensite, but the composition ratio of ferrite and martensite, or ferrite and tempered martensite may be constant or different, and may have both the part having the constant structure ratio and the part having the inclined structure ratio in the same manner as in Step 1 in Steps 2 to 4.

Step 2

The Step 2 is a step of pressing and cooling the entire steel plate after Step 1 such that the first region is transformed into martensite and such that the second region is transformed into ferrite and martensite;

In the Step 2, the first region in which the metallographic structure is heated into austenite through Step 1 is subjected to press cooling, so that the metallographic structure of the first region is transformed into martensite. The second region in which the metallographic structure is in a state in which ferrite and austenite are mixed through Step 1 is subjected to press cooling, so that the metallographic structure is changed to a state in which ferrite and martensite are mixed. The press cooling is an operation in which pressing is performed by a pressing die and cooling is performed in the pressing die. A specific method of press cooling is not particularly limited, and a publicly known method can be used. Cooling temperature and cooling rate in the press cooling are not particularly limited as long as they are in a range in which quenching is possible.

Fig. 1C illustrates an example of a state immediately after the press cooling in the Step 2 is performed. The steel plate 11 after Step 1 is pressed by a pressing die 20 and cooled, and a steel plate 12 including a first region 1B in which a metallographic structure is martensite M and a second region 2B in which a metallographic structure is ferrite F and martensite M is obtained.

Step 3

The Step 3 is a step of reheating the steel plate after the Step 2 such that the second region is transformed into ferrite and tempered martensite, a region 1P, which is a first part of the first region adjacent to the second region, is transformed into tempered martensite, and such that a region 1Q, a second part of the first region other than the region 1P, is transformed into austenite.

In Step 3, through the Step 2, the steel plate including the first region in which the metallographic structure is martensite and the second region in which ferrite and martensite are mixed is heated again to set the metallographic structure of the second region to a state in which ferrite and tempered martensite are mixed, to turn a metallographic structure of the region 1P which is a part of the first region and is adjacent to the second region into tempered martensite, and to transform a metallographic structure of the region 1Q which is another part of the first region other than the region 1P into austenite.

Fig. 1D illustrates a state in which Step 3 is performed, a metallographic structure of a second region 2C which is a part of a steel plate 13 contains ferrite F and tempered martensite M_T, and the metallographic structure of the region 1P (1PA in Fig. 1D) which is a part of the first region and is adjacent to the second region 2C is tempered martensite M_T, and the metallographic structure of the region 1Q (1QA in Fig. 1D) which is another part of the first region other than the region 1P is austenite γ.

The heating in Step 3 is preferably performed by changing a maximum temperature reached by heating each part of the steel plate.
The maximum temperature to which the region 1Q is heated in Step 3 is preferably equal to or higher than the A₃ point, and is preferably, for example, 850°C to 950°C. The maximum temperature to which the second region and the region 1P are heated in Step 3 is preferably equal to or higher than 400°C but lower than the austenite transformation starting temperature (A₁ point). By adjusting the maximum temperature to which each of the second region and the region 1P are heated in Step 3, the respective hardness of the second region and the region 1P in the hot stamped product can be adjusted.

For the heating in Step 3, heating time from the start of heating to the completion of heating is preferably within 30 seconds. By setting the heating in Step 3 as rapid short time heating in this way, productivity can be improved.

A heating method in Step 3 is not particularly limited. Examples of the heating method include furnace heating and resistance heating (such as induction heating and direct resistance heating), but resistance heating is preferable, and direct resistance heating is more preferable due to a fact that the rapid short time heating is easy. A specific method of heating is not particularly limited, and a publicly known method can be used.

In Step 3, a specific method for changing the maximum temperatures reached by heating in the second region, the region 1P, and the region 1Q is not particularly limited. In particular, the specific method is preferably performed by electrically heating the entire steel plate and cooling the second region and the region 1P. In this case, a method of cooling the second region and the region 1P is not particularly limited, and is preferably performed by, for example, spraying a cooling medium (for example, a cooling gas) or bringing a die into contact. Specific methods of the method of spraying the cooling medium and the method of bringing the die into contact is not particularly limited, and a publicly known method can be used. Further, each of the adjustments of the maximum temperatures of the second region and the region 1P reached by heating can also be performed independently, but in view of a simpler process, the adjustments are preferably performed at the same time as a single operation (that is, both the maximum temperatures reached by heating are collectively adjusted without distinguishing between the second region and the region 1P).

Step 4

The Step 4 is a step of pressing and recooling the entire steel plate after Step 3 such that the region 1Q is transformed into martensite and such that the steel plate is pressed into a desired shape.

Through Step 3, the region 1Q in which the metallographic structure is austenite is quenched in Step 4 and transformed into martensite. Through Step 3, the region 1P in which the metallographic structure is tempered martensite is tempered martensite even after Step 4. Through Step 3, the second region in which the metallographic structure is in a state in which ferrite and tempered martensite are mixed is in a state in which ferrite and tempered martensite are mixed even after Step 4.

Fig. 1E illustrates an example of a state immediately after the press cooling in Step 4 is performed. The steel plate after Step 3 is pressed by a pressing die 21 and cooled, and becomes a hot stamped product 14. In the hot stamped product 14, the metallographic structure of the region 1Q (1QB in Fig. 1E) is martensite M, the metallographic structure of the region 1P (1PB in Fig. 1E) is tempered martensite M_T, and the metallographic structure of the second region 2D is in a state in which ferrite F and tempered martensite M_T are mixed. The region 1Q of the hot stamped product 14 is a part (hard portion) having high strength containing martensite, and the region 1P and the second region are parts (soft portion) having lower strength than that of the hard portion. The soft portion is easy to be subjected to post processing such as piercing, trimming, and welding.

In the press cooling in Step 4, the steel plate is pressed into a desired shape to form the hot stamped product. A specific method of press cooling is not particularly limited, and a publicly known method can be used. Cooling temperature and cooling rate in the press cooling are not particularly limited as long as they are in a range in which quenching of the first region is possible. The final shape of the hot stamped product is not particularly limited. For example, the shape may be a flat plate as illustrated in Fig. 1E, and a desired shape can be used depending on applications, specifications, and the like of the hot stamped product, such as a shape in which a cross section is a hat shape as illustrated in Fig. 6.

Third Region

In the hot stamping method of the present invention, the steel plate may include a third region in addition to the first region, the second region, the region 1P, and the region 1Q. In the hot stamping method of the present invention, the third region may be a region in which the metallographic structure does not change through Steps 1 to 4. For example, when the metallographic structure of the steel plate before Step 1 includes ferrite and cementite, the third region may be a region containing ferrite and cementite through Steps 1 to 4.

A preferable aspect in the case of including the third region is as follows.

That is, it is preferable that the steel plate includes the third region which is a part of the steel plate, the third region is adjacent to the second region, but is not adjacent to the first region, the region 1P, and the region 1Q, and the third region has a ferrite and cementite structure during Steps 1 to 4. In this case, it is preferable that the maximum temperature to which the third region is heated in Step 1 and Step 3 is lower than the austenite transformation starting temperature (A₁ point). A specific method for adjusting the maximum temperature of the third region which is reached by heating in Step 1 and Step 3 is not particularly limited. In particular, the specific method is preferably performed by electrically heating the entire steel plate and cooling the second region, the region 1P, and also the third region. In this case, a method of cooling the third region is not particularly limited, and is preferably performed by, for example, spraying a cooling medium (for example, a cooling gas) or bringing a die into contact. Specific methods of the method of spraying the cooling medium and the method of bringing the die into contact is not particularly limited, and a publicly known method can be used.

As described above, examples of the metallographic structure containing ferrite and cementite include a metallographic structure containing ferrite and cementite, a metallographic structure containing ferrite and pearlite, and a metallographic structure containing ferrite, cementite, and pearlite.

Figs. 2A to 2E illustrate another example of the hot stamping method in the case where the steel plate includes the third region.

The steel plate 40 of Fig. 2A illustrates a steel plate in a state before Step 1 is performed. A metallographic structure of the steel plate 40 in Fig. 2A contains ferrite F and pearlite P. Fig. 2B illustrates a steel plate 41 after performing Step 1, and is the same as Fig. 1B except that there is a third region 3A in which the metallographic structure maintains ferrite F and pearlite P. The third region 3A is adjacent to the second region 2E and is not adjacent to the first region 1C. Fig. 2C schematically illustrates a state after performing the Step 2, and is the same as Fig. 1C except that there is a third region 3B containing ferrite F and pearlite P. Fig. 2D illustrates a steel plate after performing Step 3, and is the same as Fig. 1D except that there is a third region 3C containing ferrite F and pearlite P. Fig. 2Eschematically illustrates a state immediately after performing press cooling of Step 4, and is the same as Fig. 1E except that there is a third region 3D containing ferrite F and pearlite P. In this case, in a hot stamped product 44 obtained after Step 4, the third region 3D is also a soft portion.

As described above, examples of the metallographic structure of the steel plate in a state before performing Step 1 include a metallographic structure containing ferrite and cementite and a metallographic structure containing ferrite, cementite, and pearlite besides a metallographic structure containing ferrite and pearlite illustrated in Figs. 2A to 2E. The same applies to the metallographic structure of the third region.

As described above, in the hot stamping method of the present invention, the heating in Step 1 and the heating in Step 3 are preferably within 30 seconds from the start of heating to the completion of heating separately. In this way, by setting the heating in Step 1 and the heating in Step 3 as rapid short time heating in a short time, not only productivity is improved, but characteristics of the obtained hot stamped product can be improved. That is, in the region 1Q included in the first region of the hot stamped product obtained by performing the rapid short time heating as the heating in Step 1 and the heating in Step 3, since processing referred to as quenching from the rapid short time heating is performed twice in Step 1 to the Step 2 and Step 3 to Step 4, the metallographic structure is fine martensite (a grain size number measured in accordance with JIS G0551:2013 is equal to or greater than 10). It is known that when the crystal grain size of the metallographic structure is fine, strength of the metallographic structure is improved.

Hot Stamped Product

A hot stamped product according to an aspect of the present invention includes the region 1Q containing martensite, the region 1P containing tempered martensite adjacent to the region 1Q, the second region containing ferrite and tempered martensite adjacent to the region 1P and not adjacent to the region 1Q, and a grain size number of martensite in the region 1Q is equal to or greater than 10.

The hot stamped product 14 of Fig. 1E schematically illustrates an example of the hot stamped product of the present invention. The hot stamped product 14 includes the region 1Q (1QB in Fig. 1E) containing martensite M, the region 1P (1PB in Fig. 1E) containing tempered martensite M_T, and the second region (2D in Fig. 1E) containing ferrite F and tempered martensite M_T, and the grain size number of martensite in the region 1Q is equal to or greater than 10.

In the hot stamped product of the present invention, the second region is in a state in which ferrite and tempered martensite are mixed, but may be a second region in which a composition ratio of ferrite and austenite in the second region is constant in the longitudinal direction (constant structure ratio) or different. For example, the composition of the second region may be a composition in which a proportion of tempered martensite gradually increases from a part far from the region 1P toward a part close to the region 1P (inclined structure ratio). In addition, the composition of the second region may have both a part having a constant structure ratio and a part having an inclined structure ratio.

Preferably, in the hot stamping method of the present invention, the hot stamped product of the present invention can be obtained by separately performing the heating in Step 1 and the heating in Step 3 when the heating time from the start of heating to the completion of heating is within 30 seconds.

In the hot stamped product of the present invention, parts corresponding to the region 1Q, the region 1P, and the second region may be only one portion separately, or may be a plurality of portions separately. The hot stamped product of the present invention may have a part other than the region 1Q, the region 1P, and the second region. An example of a preferable aspect of the hot stamped product of the present invention in the case of including a part other than the region 1Q, the region 1P, and the second region includes an aspect including the third region containing ferrite and cementite adjacent to the second region and not adjacent to the region 1P and the region 1Q. In this case, the part corresponding to the third region may be only one portion, or may be a plurality of portions. A diagram of an example of the hot stamped product according to the aspect is the hot stamped product 44 of Fig. 2E. The hot stamped product 44 of Fig. 2E is the same as the hot stamped product 14 of Fig. 1E except that the hot stamped product 44 includes the third region 3D containing ferrite F and pearlite P.

In the hot stamped product of the present invention, the region 1Q is a hard portion, and the second region and the region 1P are soft portions. In addition, when the third region includes a region containing ferrite and cementite, the third region is a soft portion.

In the hot stamped product of the present invention, the grain size number measured in accordance with JIS G0551:2013 of martensite in the region 1Q is equal to or greater than 10, preferably equal to or greater than 11, and more preferably equal to or greater than 11.5.

The hot stamped product of the present invention is not limited in the form or application, but can be used as a vehicle body, a bumper, an oil pan, an inner panel, a pillar (such as A-pillar, B-pillar, C-pillar, and D-pillar), a wheel house, or the like.

Hereinafter, the present invention will be described in more detail based on Examples, but the scope of the present invention is not interpreted limitedly by the following Examples.

Step 1

A steel plate of 1500 MPa grade suitable for hot stamping was suspended between left and right electrodes of a direct resistance heating apparatus, and the steel plate was sandwiched between upper and lower electrodes to apply current between the left and right electrodes. Here, the first region, which is a part of the steel plate, was heated from room temperature to 900°C for 20 seconds. The second region, which is adjacent to the first region and is another part of the steel plate, was electrically heated by controlling the maximum temperature reached by heating of the second region so as to reach 760°C in 20 seconds from room temperature. By the direct resistance heating, the first region became austenite, and the second region became a region containing ferrite and austenite.

Step 2

The application of the current was stopped when each region reached a predetermined temperature in Step 1, and press cooling was immediately performed. The press cooling was performed using a pressing die provided with a water passage that guides cooling water for rapidly cooling the steel plate inside, the steel plate was quenched by rapid cooling while pressed into a flat plate, the metallographic structure of the first region was transformed into martensite, and the metallographic structure of the second region was changed to a state of containing ferrite and martensite. Press cooling time (the time during which the pressing die is held at its bottom dead point) was 5 to 10 seconds.

Step 3

The flat plate-shaped steel plate obtained in the Step 2 was electrically heated again using the direct resistance heating apparatus used in Step 1. Here, the second region, and the region 1P, which is a part of the first region and is adjacent to the second region, were electrically heated by controlling the maximum temperature reached by heating of the second region and the region 1P so as to reach 700°C in 20 seconds from room temperature. The region 1Q of the first region other than the region 1P was heated from room temperature to 900°C for 20 seconds. By the direct resistance heating, the region 1Q is a region containing austenite, the region 1P is a region containing tempered martensite, and the second region is a region containing austenite and tempered martensite.

Step 4

The application of the current was stopped when each region reached a predetermined temperature in Step 3, and press cooling was immediately performed. The press cooling was performed in the same manner as in the Step 2. The steel plate was rapidly cooled by press cooling while pressed into a flat plate to obtain the hot stamped product. Press cooling time (the time during which the pressing die is held at its bottom dead point) was 5 to 10 seconds. When the obtained hot stamped product is examined, the region 1Q containing martensite, the region 1P containing tempered martensite adjacent to the region 1Q, the second region containing ferrite and tempered martensite adjacent to the region 1P and not adjacent to the region 1Q are included. The second region includes a part where the composition ratio of ferrite and tempered martensite is constant in the longitudinal direction (part having the constant structure ratio) and a part where a proportion of tempered martensite gradually increases from a part far from the region 1P toward a part close to the region 1P (part having the inclined structure ratio), and the part having the inclined structure ratio was adjacent to the region 1P.

Evaluation of mechanical properties

Vickers hardness of each of the region 1Q (martensite), the region 1P (tempered martensite), and the second region (a region in which ferrite and tempered martensite are mixed) of the obtained hot stamped product was measured. Five points were measured at 300 gf (load: 300 g, HV: 0.3) in accordance with JIS Z2244:2009 using a Vickers hardness tester, and an average value thereof was determined.
As a result, the region 1Q was 523 HV, the region 1P was 273 HV, and the second region was 223 HV. From the result, in Example 1, it was found that the hot stamped product having a hard portion excellent in strength and a soft portion which is easy to be post-processed was obtained.

It was confirmed that the hardness of the second region and the region 1P, which are the soft portions, can be adjusted by changing the maximum temperature of the second region or the region 1P reached by the heating in Step 1 or Step 3 by performing a separate test. As a result, it was found that the hardness of the second region (a region in which ferrite and tempered martensite are mixed) can be adjusted in a range where a lower limit is 223 HV (about 710 MPa). It was found that the hardness of the region 1P can be adjusted in a range where a lower limit is 270 HV (about 850 MPa). From the result, it was found that the hot stamped product of the present invention can adjust properties of the soft portion according to various applications and specifications.

Measurement of Crystal Grain Size

The crystal grain size of martensite in the region 1Q of the obtained hot stamped product was measured in accordance with JIS G0551:2013. When five points were measured to determine an average value, the grain size number was 11.9 (crystal grain diameter was about 6.5 μm). For reference, when the grain size of martensite of the first region after the Step 2 and before Step 3 was measured at five points to obtain an average value in the same way, the grain size number was 11.1 (crystal grain size was about 8.5 μm). From the result, it was found that the crystal grain size of martensite in the region 1Q was made finer by performing quenching twice from rapid short time heating in Example 1.

Next, in order to evaluate toughness of the soft portion of the hot stamped product obtained by the hot stamping method of the present invention, a bending strength test (bending test) was performed according to the VDA standard (VDA 238-100) specified by Verband der Automobilindustrie (the German Association of the Automotive Industry).
A test piece used for the bending strength test was prepared as follows.

Preparation of Test Piece

A plate-shaped test piece of 60 mm × 60 mm was prepared in the same manner as in Example 1 except that the first region was not provided in Step 1 of Example 1.
The entire test piece is in a state in which ferrite and tempered martensite are mixed, and corresponds to the second region of the soft portion of the hot stamped product obtained by the hot stamping method of the present invention.

Measurement of Bending Strength

As illustrated in Fig. 3, a plate-shaped test piece 32 of 60 mm × 60 mm was placed on two support rolls 31 having a diameter of 30 mm, and a punch 33 was pressed in at a speed of 20 mm/min. A radius of curvature of a tip portion of the punch is 0.4 mm. An interval between the rolls was set to a plate thickness of the plate-shaped test piece × 2 + 0.5 mm. The bending angle (the VDA bending angle) at which a crack occurs on the test piece (at a maximum load) was determined. It illustrates that the larger the bending angle at the maximum load, the higher toughness at the time of crushing. As illustrated in Fig. 4, the bending angle is an angle determined by (180° - α) × 1/2 when an angle formed by a bent test piece 34 after the bending test is α (α is 0° to 180°).

As a result of performing the test three times to obtain an average value, the bending angle of the test piece in Example 2 was 62°.

Preparation of Test Piece

A plate-shaped test piece of 60 mm × 60 mm was prepared in the same manner as in Example 1 except that the second region was not provided in Step 1 of Example 1, and all the first region was tempered martensite in Step 3. The entire test piece is tempered martensite, and corresponds to the region 1P of the soft portion of the hot stamped product obtained by the hot stamping method of the present invention.

Measurement of Bending Strength

As a result of performing the bending test three times to obtain an average value of the bending angle in the same manner as in Example 2, the bending angle of the test piece in Example 3 was 60°.

As follows, the hot stamped product including the third region (region containing ferrite and pearlite) in addition to the region 1Q, the region 1P, and the second region was produced by using a steel plate of 1500 MPa grade suitable for hot stamping.

Step 1

A steel plate of 1500 MPa grade suitable for hot stamping was suspended between left and right electrodes of a direct resistance heating apparatus, and the steel plate was sandwiched between upper and lower electrodes to apply current between the left and right electrodes. Here, the first region, which is a part of the steel plate, was heated from room temperature to 900°C for 20 seconds. The second region, which is adjacent to the first region and is another part of the steel plate, was electrically heated by controlling a maximum temperature reached by heating of the second region so as to reach 760°C in 20 seconds from room temperature. The third region, which is adjacent to the second region and is not adjacent to the first region, was electrically heated by controlling a maximum temperature reached by heating of the third region so as to reach 650°C in 20 seconds from room temperature. By the direct resistance heating, the first region became austenite, the second region became a region containing ferrite and austenite, and the third region maintains a state of containing ferrite and pearlite.

Step 2

The application of the current was stopped when each region reached a desired temperature in Step 1, and press cooling was immediately performed. The press cooling was performed using a pressing die provided with a water passage that guides cooling water for rapidly cooling the steel plate inside, the steel plate was quenched by rapid cooling while pressed into a flat plate, the metallographic structure of the first region was transformed into martensite, and the metallographic structure of the second region was changed to a state of containing ferrite and martensite. A metallographic structure of the third region contained ferrite and pearlite. Press cooling time (the time during which the pressing die is held at its bottom dead point) was 5 to 10 seconds.

Step 3

The flat plate-shaped steel plate obtained in the Step 2 was electrically heated again using the direct resistance heating apparatus used in Step 1. Here, the second region, and the region 1P, which is a part of the first region and is adjacent to the second region, were electrically heated by controlling a maximum temperature reached by heating of the region 1P so as to reach 700°C in 20 seconds from room temperature. The region 1Q of the first region other than the region 1P was heated from room temperature to 900°C for 20 seconds. The third region was electrically heated by controlling a maximum temperature reached by heating of the third region so as to reach 650°C in 20 seconds from room temperature. By the direct resistance heating, the region 1Q is a region containing austenite, the region 1P is a region containing tempered martensite, and the second region is a region containing austenite and tempered martensite. The third region is a region containing ferrite and pearlite.

Step 4

The application of the current was stopped when each region reached the predetermined temperature in Step 3, and press cooling was immediately performed. The press cooling was performed in the same manner as in the Step 2. The steel plate was rapidly cooled by press cooling while pressed into a flat plate to obtain the hot stamped product. Press cooling time (the time during which the pressing die is held at its bottom dead point) was 5 to 10 seconds. When the obtained hot stamped product was examined, the region 1Q containing martensite, the region 1P containing tempered martensite adjacent to the region 1Q, the second region containing ferrite and tempered martensite adjacent to the region 1P and not adjacent to the region 1Q, and the third region containing ferrite and pearlite adjacent to the second region and not adjacent to the region 1P and the region 1Q were included.

Evaluation of Mechanical Properties

Vickers hardness of each of the region 1Q (martensite), the region 1P (tempered martensite), the second region (a region in which ferrite and tempered martensite are mixed), and the third region (a region containing ferrite and pearlite) of the obtained hot stamped product was measured in the same manner as in Example 1.
As a result, hardness of the region 1Q, the region 1P, and the second region were the same as those in Example 1 respectively, and hardness of the third region was 200 HV. From the result, in Example 4, it was found that the hot stamped product having a hard portion excellent in strength and a soft portion which is easy to be post-processed was obtained in the same manner as in Example 1.

The hardness of the second region and the region 1P, which are the soft portions, can be adjusted by changing the maximum temperature of the second region or the region 1P reached by the heating in Step 1 or Step 3 as described above. Therefore, the hot stamped product of Example 4 can adjust properties of the soft portion according to various applications and specifications.

Measurement of Crystal Grain Size

When the crystal grain size of martensite in the region 1Q of the obtained hot stamped product was measured, the grain size number was the same as the grain size number of martensite in the region 1Q in Example 1. From the result, it was found that the crystal grain size of martensite in the region 1Q was made finer by performing quenching twice from rapid short time heating in the same manner as in Example 1 also in Example 4.

Comparative Example 1

For comparison, the hot stamped product was produced and evaluated by a method other than the hot stamping method of the present invention as illustrated below.

Production of Hot Stamped Product of Comparative Example 1

A steel plate of 1500 MPa grade suitable for hot stamping was used. The steel plate was electrically heated using a direct resistance heating apparatus. At this time, a part of the steel plate was heated to 900°C to be transformed into austenite, and the maximum temperature reached by heating the other part was controlled to a temperature below the A₁ point to maintain ferrite. Thereafter, austenite was transformed into martensite by performing press cooling and quenching. In this way, the hot stamped product of Comparative Example 1 was obtained. When the hot stamped product of Comparative Example 1 was examined, a portion thereof was martensite (this part is also referred to as "R (M) portion"), a portion thereof is ferrite and pearlite (this part is also referred to as "R (F + P) portion"), and a part in which ferrite and martensite mixed (this portion is also referred to as "R (F + M) portion") is present between the R (M) portion and the R (F + P) portion.

Vickers hardness of the R (F + M) portion of the hot stamped product of Comparative Example 1 was determined. Five points were measured at 300 gf (load: 300 g, HV: 0.3) in accordance with JIS Z2244:2009 using a Vickers hardness tester, and an average value thereof was determined. The result was 294 HV.

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 to obtain an average value of the bending angle. The bending angle of the test piece in Comparative Example 2 was 27°.

By comparing Example 2 and Example 3 with Comparative Example 2, it was found that the soft portion of the hot stamped product obtained by the hot stamping method of the present invention has excellent toughness.

As follows, the hot stamped product including the third region (region containing ferrite and cementite) in addition to the region 1Q, the region 1P, and the second region was produced by using a steel plate of 1500 MPa grade suitable for hot stamping.

Step 1

A steel plate of 1500 MPa grade suitable for hot stamping (1200 mm in length, 500 mm in width, 1 mm in thickness) was suspended between the left and right electrodes of the direct resistance heating apparatus, and the steel plate was sandwiched between the upper and lower electrodes to apply current between the left and right electrodes. Here, the first region, which is a part of the steel plate, was heated from room temperature to 900°C for 20 seconds. The second region, which is adjacent to the first region and is another part of the steel plate, was electrically heated by controlling a maximum temperature reached by heating of the second region so as to be equal to or higher than the austenite transformation starting temperature (A₁ point) but lower than the austenite transformation finishing temperature (A₃ point) in 20 seconds from room temperature. The third region, which is adjacent to the second region and is not adjacent to the first region, was electrically heated by controlling a maximum temperature reached by heating of the third region so as to reach 600°C in 20 seconds from room temperature. By the direct resistance heating, the first region became austenite, the second region became a region containing ferrite and austenite, and the third region maintains a state of containing ferrite and cementite.

A steel plate 60 of Fig. 5A illustrates a state before Step 1 is performed. A metallographic structure of the steel plate 60 in Fig. 5A contains ferrite F and cementite θ. Fig. 5B illustrates a state in which Step 1 is performed, a metallographic structure of the first region 1E which is a part of the steel plate 61 is transformed into austenite γ, and a metallographic structure of the second region 2I which is another part of the steel plate 61 and is adjacent to the first region 1E is a region containing ferrite F and austenite γ. A metallographic structure of the third region 3E which is another part of the steel plate 61 and is adjacent to the second region 2I and not adjacent to the first region 1E contains ferrite F and cementite θ.

Step 2

The application of the current was stopped when each region reached a desired temperature in Step 1, and press cooling was immediately performed. The press cooling was performed using a pressing die provided with a water passage that guides cooling water for rapidly cooling the steel plate inside, the steel plate was quenched by rapid cooling while pressed into a flat plate, the metallographic structure of the first region was transformed into martensite, and the metallographic structure of the second region was changed to a state of containing ferrite and martensite. A metallographic structure of the third region contained ferrite and cementite. Press cooling time (the time during which the pressing die is held at its bottom dead point) was 15 seconds. The first region, the third region, and the second region were provided so that the metallographic structure changes in a length direction of the steel plate. A length of the first region was 810 mm, a length of the second region was 20 mm, and a length of the third region was 370 mm. Fig. 5C illustrates a state of a steel plate 62 after the press cooling in the Step 2 was performed. The steel plate 62 includes a first region 1F in which a metallographic structure is martensite M and a second region 2J in which a metallographic structure is ferrite F and martensite M, and a third region 3F in which a metallographic structure is ferrite F and cementite θ.

Step 3

The flat plate-shaped steel plate obtained in the Step 2 was electrically heated again using the direct resistance heating apparatus used in Step 1. Here, the second region, and the region 1P, which is a part of the first region and is adjacent to the second region, were electrically heated by controlling a maximum temperature reached by heating of the region 1P so as to be equal to or higher than 600°C but lower than the austenite transformation start temperature (A1 point) in 20 seconds from room temperature. The region 1Q of the first region other than the region 1P was heated from room temperature to 900°C for 20 seconds. The third region was electrically heated by controlling a maximum temperature reached by heating of the third region so as to reach 600°C in 20 seconds from room temperature. By the direct resistance heating, the region 1Q is a region containing austenite, the region 1P is a region containing tempered martensite, and the second region is a region containing austenite and tempered martensite. The third region is a region containing ferrite and cementite. A length of the region 1Q was 800 mm, a length of the region 1P was 10 mm, a length of the second region was 20 mm, and a length of the third region was 370 mm. Fig. 5D illustrates a state in which Step 3 is performed, a metallographic structure of a second region 2K which is a part of a steel plate 63 contains ferrite F and tempered martensite M_T, and the metallographic structure of the region 1P (1PE in Fig. 5D) which is a part of the first region and is adjacent to the second region 2K is tempered martensite M_T, and the metallographic structure of the region 1Q (1QE in Fig. 5D) which is another part of the first region other than the region 1P is austenite γ. The third region 3G, which is a part of the steel plate 63, is a region containing ferrite and cementite.

Step 4

The application of the current was stopped when each region reached the predetermined temperature in Step 3, and press cooling was immediately performed. The press cooling was performed using a pressing die provided with a water passage that guides cooling water for rapidly cooling the steel plate inside, and the steel plate was quenched by rapid cooling while pressed into a shape in which a cross section in a width direction is a hat shape to obtain the hot stamped product. Press cooling time (the time during which the pressing die is held at its bottom dead point) was 15 seconds. When the obtained hot stamped product was examined, the region 1Q containing martensite, the region 1P containing tempered martensite adjacent to the region 1Q, the second region containing ferrite and tempered martensite adjacent to the region 1P and not adjacent to the region 1Q, and the third region containing ferrite and cementite adjacent to the second region and not adjacent to the region 1P and the region 1Q were included.
Fig. 5E illustrates a state of a hot stamped product 64 produced by performing the press cooling in Step 4. The steel plate after Step 3 is pressed by a pressing die (not illustrated) and cooled to become the hot stamped product 64. In the hot stamped product 64, the metallographic structure of the region 1Q (1QF in Fig. 5E) is martensite M, the metallographic structure of the region 1P (1PF in Fig. 5E) is tempered martensite M_T, the metallographic structure of the second region 2L is in a state in which ferrite F and tempered martensite M_T are mixed, and the metallographic structure of the third region 3H contains ferrite and cementite. The hot stamped product 64 has a shape in which a cross section in the width direction is a hat shape. Fig. 6 illustrates a perspective view of the hot stamped product 64.

Evaluation of Mechanical Properties

Vickers hardness of each of the region 1Q (martensite), the region 1P (tempered martensite), the second region (a region in which ferrite and tempered martensite are mixed), and the third region (a region containing ferrite and cementite) of the obtained hot stamped product was measured in the same manner as in Example 1. As a result, the region 1Q was 450 HV to 500 HV (1500 MPa to 1700 MPa), the region 1P was about 280 HV (about 890 MPa), the second region was 200 HV to 280 HV (640 MPa to 890 MPa), and the third region was 170 HV to 200 HV (550 MPa to 640 MPa). From the result, in Example 5, it was found that the hot stamped product having a hard portion excellent in strength and a soft portion which is easy to be post-processed was obtained in the same manner as in Example 1.

The hardness of the second region and the region 1P, which are the soft portions, can be adjusted by changing the maximum temperature of the second region or the region 1P reached by the heating in Step 1 or Step 3 as described above. Therefore, the hot stamped product of Example 5 can adjust properties of the soft portion according to various applications and specifications.

Measurement of Crystal Grain Size

When the crystal grain size of martensite in the region 1Q of the obtained hot stamped product was measured, the grain size number was 11.7. From the result, it was found that the crystal grain size of martensite in the region 1Q was made finer by performing quenching twice from rapid short time heating in the same manner as in Example 1 even in Example 5.

This application claims priority to Japanese Patent Application No. 2018-196982 filed on October 18, 2018 and Japanese Patent Application No.2019-160717 filed on September 3, 2019, the entire contents of which are incorporated herein by reference.

Claims

A hot stamping method comprising:
heating a steel plate such that a first region of the steel plate is transformed into austenite and such that a second region of the steel plate other than the first region is transformed into ferrite and austenite, the second region being adjacent to the first region;
pressing and cooling the entire steel plate after the heating such that the first region is transformed into martensite and such that the second region is transformed into ferrite and martensite;
reheating the steel plate after the pressing and cooling such that the second region is transformed into ferrite and tempered martensite, such that a first part of the first region adjacent to the second region is transformed into tempered martensite, and such that a second part of the first region other than the first part is transformed into austenite; and
pressing and recooling the entire steel plate after the reheating such that the second part of the first region is transformed into martensite and such that the steel plate is pressed into a desired shape.
The hot stamping method according to claim 1, wherein each of the heating and the reheating is not longer than 30 seconds.
The hot stamping method according to claim 1 or 2, wherein, in each of the heating and the reheating, the steel plate is heated by resistance heating.
The hot stamping method according to any one of claims 1 to 3, wherein a maximum temperature to which the steel plate is heated in the heating is different between the first region and the second region.
The hot stamping method according to any one of claims 1 to 4, wherein a maximum temperature to which the steel plate is heated in the reheating is different between the second part of the first region and the second region.
The hot stamping method according to any one of claims 1 to 5, wherein the maximum temperature to which the second region is heated in the heating is equal to or higher than an austenite transformation starting temperature but lower than an austenite transformation finishing temperature.
The hot stamping method according to any one of claims 1 to 6, wherein the maximum temperature to which the second region and the first part of the first region are heated in the reheating is equal to or higher than 400°C but lower than the austenite transformation starting temperature.
The hot stamping method according to any one of claims 1 to 7, wherein the steel plate has a third region, the third region being adjacent to the second region but not adjacent to the first region, and
wherein the third region has a ferrite and cementite structure during the heating, the pressing and cooling, the reheating, and the pressing and recooling.
The hot stamping method according to claim 8, wherein the maximum temperature to which the third region is heated in each of the heating and the reheating is below the austenite transformation starting temperature.
A hot stamped product comprising:
a first region including a first part having a tempered martensite structure and a second part having a martensite structure, the second part being adjacent to the first part; and
a second region having a ferrite and tempered martensite structure, the second region being adjacent to the first part of the first region but not adjacent to the second part of the first region,
wherein a grain size number of the martensite structure in the second part of the first region is equal to or greater than 10 in accordance with JIS G0551:2013.
The hot stamped product according to claim 10, further comprising a third region having a ferrite and cementite structure, the third region being adjacent to the second region but not adjacent to the first region.