WO2020080486A1 - Hot stamping method and hot stamped product - Google Patents

Hot stamping method and hot stamped product Download PDF

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Publication number
WO2020080486A1
WO2020080486A1 PCT/JP2019/040964 JP2019040964W WO2020080486A1 WO 2020080486 A1 WO2020080486 A1 WO 2020080486A1 JP 2019040964 W JP2019040964 W JP 2019040964W WO 2020080486 A1 WO2020080486 A1 WO 2020080486A1
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WO
WIPO (PCT)
Prior art keywords
region
steel plate
heating
martensite
stamped product
Prior art date
Application number
PCT/JP2019/040964
Other languages
French (fr)
Inventor
Yasuyuki Koyata
Hironori Ooyama
Maho Hosogi
Hirotaka Tanaka
Yoshitaka Misaka
Nobuyuki EHARA
Toshihiro Minagawa
Original Assignee
Neturen Co., Ltd.
Arrk Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from JP2019160716A external-priority patent/JP7372787B2/en
Application filed by Neturen Co., Ltd., Arrk Corporation filed Critical Neturen Co., Ltd.
Priority to DE112019004805.8T priority Critical patent/DE112019004805T5/en
Publication of WO2020080486A1 publication Critical patent/WO2020080486A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/34—Methods of heating
    • C21D1/40—Direct resistance heating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2221/00—Treating localised areas of an article

Definitions

  • the present invention relates to a hot stamping method and a hot stamped product.
  • 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.
  • hot stamped products have high strength by being quenched.
  • 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).
  • 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.
  • a hot stamping method includes heating a steel plate such that the entire steel plate is transformed into austenite, pressing and cooling the entire steel plate after the heating such that the entire steel plate is transformed into martensite, reheating the steel plate after the pressing and cooling 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 tempered martensite, and pressing and recooling the entire steel plate after the reheating such that the first region is transformed into martensite and such that the steel plate is pressed into a desired shape.
  • a hot stamped product includes a first region having a martensite and a second region having a tempered martensite structure.
  • the grain size number of the martensite structure in 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. 2 is a diagram illustrating a bending test according to the VDA standard.
  • Fig. 3 is a diagram illustrating a VDA bending angle.
  • Fig. 4A is a diagram illustrating another example of a steel plate.
  • Fig. 4B is a diagram illustrating another example of a heated steel plate.
  • FIG. 4C is a diagram illustrating another example of a press cooled steel plate.
  • Fig. 4D is a diagram illustrating another example of a reheated steel plate.
  • Fig. 4E is a diagram illustrating another example of a hot stamped product.
  • Fig. 5 is a diagram illustrating an example of a hot stamped product produced by the hot stamping method illustrated in Figs. 4A to 4E.
  • a hot stamping method include: Step 1 - heating a steel plate such that the entire steel plate is transformed into austenite; Step 2 - pressing and cooling the entire steel plate after the heating such that the entire steel plate is transformed into martensite; Step 3 - reheating the steel plate after the pressing and cooling 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 tempered martensite; and Step 4- pressing and recooling the entire steel plate after the reheating such that 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 ⁇ .
  • Examples of the metallographic structure of the steel plate 10 include a metallographic structure containing ferrite F and cementite ⁇ , a metallographic structure containing ferrite F and pearlite P, and a metallographic structure containing ferrite F, cementite ⁇ , and pearlite P.
  • a composition of the steel plate is not particularly limited as long as the steel plate can be quenched.
  • Step 1 is a step of heating the steel plate such that the entire steel plate is transformed into austenite.
  • Fig. 1B illustrates a state in which Step 1 is performed so that the metallographic structure of the entire steel plate 11 is austenite ⁇ .
  • the maximum temperature to which the steel plate is heated in Step 1 is preferably equal to or higher than the A 3 point - a temperature at which transformation from ferrite to austenite is completed, and is preferably, for example, 850°C to 950°C.
  • heating time from the start of heating to the completion of heating is preferably within 30 seconds, i.e. not longer than 30 seconds.
  • 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.
  • a shape of the steel plate is not particularly limited as long as it is a plate shape.
  • the steel plate is preferably a flat plate.
  • 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.
  • Step 2 is a step of pressing and cooling the entire steel plate after Step 1 such that the entire steel plate is transformed into austenite.
  • Step 2 the steel plate in which the entire metallographic structure is heated into austenite through Step 1 is subjected to press cooling, so that the metallographic structure of the entire steel plate is transformed into martensite.
  • 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 Step 2 is performed.
  • the steel plate 11 after Step 1 is pressed by a pressing die 20, and a steel plate 12 in which a metallographic structure is martensite M is obtained.
  • Step 3 is a step of reheating the steel plate after Step 2 such that a first region of the steel plate is transformed into austenite and such that a second region of the steel plate is transformed into tempered martensite.
  • Step 3 the steel plate in which the whole metallographic structure is martensite is heated again, a part of the steel plate is transformed into austenite, and another part of the steel plate is turned into tempered martensite.
  • Step 3 a region (third region) including austenite and tempered martensite may be formed between the first region (austenite) and the second region (tempered martensite).
  • Fig. 1D illustrates a state in which Step 3 is performed so that the metallographic structure of the first region 1A which is a part of the steel plate 13 is austenite ⁇ , and the metallographic structure of the second region 2A which is another part of the steel plate is tempered martensite M T .
  • 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 first region is heated in Step 3 is preferably equal to or higher than the A 3 point, and is preferably, for example, 850°C to 950°C.
  • the maximum temperature to which the second region is heated in Step 3 is not particularly limited as long as the temperature can be tempered, but is preferably equal to or higher than 400°C but lower than the austenite transformation starting temperature (A 1 point).
  • the width of the third region can be adjusted, for example, by adjusting a temperature difference between the maximum temperature of the first region and the maximum temperature of the second region, which are reached by the heating in Step 3, or by adjusting heating time.
  • the width of the third region can be narrowed by reducing the temperature difference or shortening the heating time.
  • heating time from the start of heating to the completion of heating is preferably within 30 seconds.
  • 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.
  • a specific method for changing the maximum temperature reached by heating in the first region and the second region is not particularly limited.
  • the specific method is preferably performed by electrically heating the entire steel plate and cooling the second region.
  • 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.
  • a cooling medium for example, a cooling gas
  • 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.
  • Step 4 is a step of pressing and recooling the entire steel plate after Step 3 such that the first region is transformed into martensite and such that the steel plate is pressed into a desired shape.
  • Step 3 the first region in which the metallographic structure is austenite is quenched in Step 4 and transformed into martensite.
  • Step 3 the second region in which the metallographic structure is tempered martensite is tempered martensite even after Step 4.
  • the third region is a region including martensite and tempered martensite 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.
  • the metallographic structure of the first region 1B is martensite M
  • the metallographic structure of the second region 2B is tempered martensite M T .
  • the first region 1B of the hot stamped product 14 is a part (hard portion) having high strength containing martensite
  • the second region 2B is a part (soft portion) having lower strength than that of the hard portion containing tempered martensite.
  • the soft portion is easy to be subjected to post processings such as piercing, trimming, and welding.
  • 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.
  • 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. 5E.
  • 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.
  • 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.
  • 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.
  • 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.
  • a hot stamped product according to an aspect of the present invention has a first region containing martensite and a second region containing tempered martensite, and a grain size number of martensite in the first region 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 first region 1B containing the martensite M and the second region 2B containing the tempered martensite M T , and the grain size number of martensite in the first region is equal to or greater than 10.
  • 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.
  • parts corresponding to the first region 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 or may not have a part other than the first region and the second region.
  • Examples of the part other than the first region and the second region include a third region including martensite and tempered martensite.
  • the hot stamped product of the present invention has the third region, the third region is preferably provided between the first region and the second region. That is, the hot stamped product of the present invention may include only the first region and the second region, or may include the first region, the second region, and the third region.
  • the first region is a hard portion
  • the second region is a soft portion.
  • the grain size number measured based on JIS G0551:2013 of martensite in the first region 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.
  • 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.
  • the entire steel plate was heated from room temperature to 900°C for 20 seconds, and the metallographic structure was transformed from ferrite and pearlite to austenite.
  • the application of the current was stopped when the temperature of the steel plate reached 900°C, 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, and the metallographic structure of the entire steel plate was transformed into martensite.
  • Press cooling time (the time during which the pressing die is held at its bottom dead point) was 5 to 10 seconds.
  • the flat plate-shaped steel plate obtained in Step 2 was electrically heated again using the direct resistance heating apparatus used in Step 1.
  • 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 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 700°C in 20 second from room temperature.
  • the direct resistance heating the first region became austenite, and the second region became tempered martensite.
  • Step 3 the application of the current was stopped when a temperature of the first region of the steel plate reached 900°C and a temperature of the second region reached 700°C, and press cooling was performed immediately.
  • the press cooling was performed in the same manner as in 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.
  • Vickers hardness of each of the first region (martensite) and the second region (tempered martensite) 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 first region was 523 HV and the second region was 273 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.
  • the hardness of the second region which is the soft portion
  • the hardness of the second region can be adjusted by changing the maximum temperature of the second region reached by the heating in Step 3 by performing a separate test.
  • the hardness of the second region 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.
  • the crystal grain size of martensite in the first region of the obtained hot stamped product was measured based on 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 ⁇ m). For reference, when the grain size of martensite of the steel plate after 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 ⁇ m). From the result, it was found that the crystal grain size of martensite in the first region was made finer by performing quenching twice from rapid short time heating in Example 1.
  • 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).
  • VDA 238-100 the VDA standard
  • Verband der Automobilindustrie the German Association of the Automotive Industry
  • 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 3 of Example 1.
  • the entire test piece is tempered martensite, and corresponds to the soft portion of the hot stamped product obtained by the hot stamping method of the present invention.
  • a plate 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.
  • the bending angle is an angle determined by (180° - ⁇ ) ⁇ 1/2 when an angle formed by a bent test piece 34 after the bending test is ⁇ ( ⁇ is 0° to 180°).
  • the bending angle of the test piece in Example 2 was 60°.
  • the hot stamped product was produced and evaluated by a method other than the hot stamping method of the present invention as illustrated below.
  • 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.
  • 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 A 1 point to maintain ferrite.
  • austenite was transformed into martensite by performing press cooling and quenching. In this way, the hot stamped product of Comparative Example 1 was obtained.
  • 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 Z 2244:2009 using a Vickers hardness tester, and an average value thereof was determined. The result was 294 HV.
  • 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°.
  • Example 2 By comparing Example 2 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.
  • 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.
  • the entire steel plate was heated from room temperature to 900°C for 20 seconds, and the metallographic structure was transformed from ferrite and cementite to austenite.
  • a steel plate 40 of Fig. 4A illustrates a state before Step 1 is performed.
  • a metallographic structure of the steel plate 40 in Fig. 4A contains ferrite F and cementite ⁇ .
  • Fig. 4B illustrates a state in which Step 1 is performed, and the metallographic structure of the entire steel plate 41 is austenite ⁇ .
  • Fig. 4C illustrates a state of a steel plate 42 after the press cooling in Step 2 was performed. A metallographic structure of the entire steel plate 42 is martensite M.
  • the flat plate-shaped steel plate obtained in Step 2 was electrically heated again using the direct resistance heating apparatus used in Step 1.
  • 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 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 600°C in 20 seconds from room temperature.
  • the direct resistance heating the first region is a region containing austenite
  • the second region is a region containing tempered martensite
  • the third region between the first region and the second region is a region containing austenite and tempered martensite.
  • 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 800 mm
  • a length of the third region was 20 mm
  • a length of the second region was 380 mm.
  • Fig. 4D illustrates a state in which Step 3 is performed, the metallographic structure of the first region 1A which is a part of a steel plate 43 is austenite ⁇ , and the metallographic structure of the second region 2C which is another part of the steel plate is tempered martensite M T , and the metallographic structure of the third region 3C present between the first region and the second region contains austenite ⁇ and tempered martensite M T .
  • Step 3 the application of the current was stopped when a temperature of the first region of the steel plate reached 900°C and a temperature of the second region reached 600°C, and press cooling was performed immediately.
  • 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.
  • Fig. 4E illustrates a state of a hot stamped product 44 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 44.
  • a metallographic structure of a first region 1D is martensite M
  • a metallographic structure of a second region 2D is tempered martensite M T
  • a metallographic structure of a third region 3D contains martensite M and tempered martensite M T .
  • the hot stamped product 44 has a shape in which a cross section in the width direction is a hat shape.
  • Fig. 5 illustrates a perspective view of the hot stamped product 44.
  • the crystal grain size of martensite in the first region 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 11.8. From the result, it was found from the result that the crystal grain size of martensite in the first region was made finer by performing quenching twice from rapid short time heating in the same manner as in Example 1 also in Example 3.

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Abstract

A hot stamping method includes heating a steel plate such that the entire steel plate is transformed into austenite, pressing and cooling the entire steel plate after the heating such that the entire steel plate is transformed into martensite, reheating the steel plate after the pressing and cooling 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 tempered martensite, and pressing and recooling the entire steel plate after the reheating such that the first region is transformed into martensite and such that the steel plate is pressed into a desired shape. The grain size number of the martensite structure in the first region of the hot stamped product is equal to or greater than 10 in accordance with JIS G0551:2013.

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 the entire steel plate is transformed into austenite, pressing and cooling the entire steel plate after the heating such that the entire steel plate is transformed into martensite, reheating the steel plate after the pressing and cooling 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 tempered martensite, and pressing and recooling the entire steel plate after the reheating such that 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 includes a first region having a martensite and a second region having a tempered martensite structure. The grain size number of the martensite structure in 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. 2 is a diagram illustrating a bending test according to the VDA standard. Fig. 3 is a diagram illustrating a VDA bending angle. Fig. 4A is a diagram illustrating another example of a steel plate. Fig. 4B is a diagram illustrating another example of a heated steel plate. Fig. 4C is a diagram illustrating another example of a press cooled steel plate. Fig. 4D is a diagram illustrating another example of a reheated steel plate. Fig. 4E is a diagram illustrating another example of a hot stamped product. Fig. 5 is a diagram illustrating an example of a hot stamped product produced by the hot stamping method illustrated in Figs. 4A to 4E.
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 the entire steel plate is transformed into austenite;
Step 2 - pressing and cooling the entire steel plate after the heating such that the entire steel plate is transformed into martensite;
Step 3 - reheating the steel plate after the pressing and cooling 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 tempered martensite; and
Step 4- pressing and recooling the entire steel plate after the reheating such that 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 the entire steel plate is transformed into austenite. Fig. 1B illustrates a state in which Step 1 is performed so that the metallographic structure of the entire steel plate 11 is austenite γ.
The maximum temperature to which the steel plate is heated in Step 1 is preferably equal to or higher than the A3 point - a temperature at which transformation from ferrite to austenite is completed, and is preferably, for example, 850°C to 950°C.
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.
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.
Step 2
Step 2 is a step of pressing and cooling the entire steel plate after Step 1 such that the entire steel plate is transformed into austenite.
In Step 2, the steel plate in which the entire metallographic structure is heated into austenite through Step 1 is subjected to press cooling, so that the metallographic structure of the entire steel plate is transformed into martensite. 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 Step 2 is performed. The steel plate 11 after Step 1 is pressed by a pressing die 20, and a steel plate 12 in which a metallographic structure is martensite M is obtained.
Step 3
Step 3 is a step of reheating the steel plate after Step 2 such that a first region of the steel plate is transformed into austenite and such that a second region of the steel plate is transformed into tempered martensite.
In Step 3, through Step 2, the steel plate in which the whole metallographic structure is martensite is heated again, a part of the steel plate is transformed into austenite, and another part of the steel plate is turned into tempered martensite. In Step 3, a region (third region) including austenite and tempered martensite may be formed between the first region (austenite) and the second region (tempered martensite).
Fig. 1D illustrates a state in which Step 3 is performed so that the metallographic structure of the first region 1A which is a part of the steel plate 13 is austenite γ, and the metallographic structure of the second region 2A which is another part of the steel plate is tempered martensite MT.
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 first region 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 is heated in Step 3 is not particularly limited as long as the temperature can be tempered, but is preferably equal to or higher than 400°C but lower than the austenite transformation starting temperature (A1 point). By adjusting the maximum temperature of the second region which is reached by the heating in Step 3, hardness of the second region can be adjusted in the obtained hot stamped product. The width of the third region can be adjusted, for example, by adjusting a temperature difference between the maximum temperature of the first region and the maximum temperature of the second region, which are reached by the heating in Step 3, or by adjusting heating time. For example, the width of the third region can be narrowed by reducing the temperature difference or shortening the heating time.
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 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.
Step 4
Step 4 is a step of pressing and recooling the entire steel plate after Step 3 such that the first region is transformed into martensite and such that the steel plate is pressed into a desired shape.
Through Step 3, the first region in which the metallographic structure is austenite is quenched in Step 4 and transformed into martensite. Through Step 3, the second region in which the metallographic structure is tempered martensite is tempered martensite even after Step 4. When the region (third region) containing austenite and tempered martensite is formed between the first region (austenite) and the second region (tempered martensite) through Step 3, the third region is a region including martensite and tempered martensite 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 first region 1B is martensite M, and the metallographic structure of the second region 2B is tempered martensite MT. The first region 1B of the hot stamped product 14 is a part (hard portion) having high strength containing martensite, and the second region 2B is a part (soft portion) having lower strength than that of the hard portion containing tempered martensite. The soft portion is easy to be subjected to post processings 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. 5E.
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 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 a processing referred to as quenching from the rapid short time heating is performed twice in Step 1 to Step 2 and Step 3 to Step 4, the metallographic structure is fine martensite (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 has a first region containing martensite and a second region containing tempered martensite, and a grain size number of martensite in the first region 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 first region 1B containing the martensite M and the second region 2B containing the tempered martensite MT, and the grain size number of martensite in the first region is equal to or greater than 10.
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 first region 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 or may not have a part other than the first region and the second region. Examples of the part other than the first region and the second region include a third region including martensite and tempered martensite. When the hot stamped product of the present invention has the third region, the third region is preferably provided between the first region and the second region. That is, the hot stamped product of the present invention may include only the first region and the second region, or may include the first region, the second region, and the third region. In the hot stamped product of the present invention, the first region is a hard portion, and the second region is a soft portion.
In the hot stamped product of the present invention, the grain size number measured based on JIS G0551:2013 of martensite in the first region 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. The entire steel plate was heated from room temperature to 900°C for 20 seconds, and the metallographic structure was transformed from ferrite and pearlite to austenite.
Step 2
The application of the current was stopped when the temperature of the steel plate reached 900°C, 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, and the metallographic structure of the entire steel plate was transformed into 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 Step 2, the entirety of which is martensite, was electrically heated again using the direct resistance heating apparatus used in Step 1. 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 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 700°C in 20 second from room temperature. By the direct resistance heating, the first region became austenite, and the second region became tempered martensite.
Step 4
In Step 3, the application of the current was stopped when a temperature of the first region of the steel plate reached 900°C and a temperature of the second region reached 700°C, and press cooling was performed immediately. The press cooling was performed in the same manner as in 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 metallographic structure of the first region was martensite, and the metallographic structure of the second region was tempered martensite.
Evaluation of Mechanical Properties
Vickers hardness of each of the first region (martensite) and the second region (tempered martensite) 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 first region was 523 HV and the second region was 273 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, which is the soft portion, can be adjusted by changing the maximum temperature of the second region reached by the heating in Step 3 by performing a separate test. As a result, it was found that the hardness of the second region 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 first region of the obtained hot stamped product was measured based on 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 steel plate after 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 first region 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 3 of Example 1. The entire test piece is tempered martensite, and corresponds to 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. 2, a plate 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. 3, 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 60°.
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 Z 2244:2009 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 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.
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. The entire steel plate was heated from room temperature to 900°C for 20 seconds, and the metallographic structure was transformed from ferrite and cementite to austenite. A steel plate 40 of Fig. 4A illustrates a state before Step 1 is performed. A metallographic structure of the steel plate 40 in Fig. 4A contains ferrite F and cementite θ. Fig. 4B illustrates a state in which Step 1 is performed, and the metallographic structure of the entire steel plate 41 is austenite γ.
Step 2
The application of the current was stopped when the temperature of the steel plate reached 900°C, 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, and the metallographic structure of the entire steel plate was transformed into martensite. Press cooling time (the time during which the pressing die is held at its bottom dead point) was 15 seconds. Fig. 4C illustrates a state of a steel plate 42 after the press cooling in Step 2 was performed. A metallographic structure of the entire steel plate 42 is martensite M.
Step 3
The flat plate-shaped steel plate obtained in Step 2, the entirety of which is martensite, was electrically heated again using the direct resistance heating apparatus used in Step 1. 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 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 600°C in 20 seconds from room temperature. By the direct resistance heating, the first region is a region containing austenite, the second region is a region containing tempered martensite, and the third region between the first region and the second region is a region containing austenite and tempered martensite. 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 800 mm, a length of the third region was 20 mm, and a length of the second region was 380 mm. Fig. 4D illustrates a state in which Step 3 is performed, the metallographic structure of the first region 1A which is a part of a steel plate 43 is austenite γ, and the metallographic structure of the second region 2C which is another part of the steel plate is tempered martensite MT, and the metallographic structure of the third region 3C present between the first region and the second region contains austenite γ and tempered martensite MT.
Step 4
In Step 3, the application of the current was stopped when a temperature of the first region of the steel plate reached 900°C and a temperature of the second region reached 600°C, and press cooling was performed immediately. 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 metallographic structure in the first region was martensite, the metallographic structure in the second region was tempered martensite, and the metallographic structure in the third region was martensite and tempered martensite. Fig. 4E illustrates a state of a hot stamped product 44 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 44. In the hot stamped product 44, a metallographic structure of a first region 1D is martensite M, a metallographic structure of a second region 2D is tempered martensite MT, and a metallographic structure of a third region 3D contains martensite M and tempered martensite MT. The hot stamped product 44 has a shape in which a cross section in the width direction is a hat shape. Fig. 5 illustrates a perspective view of the hot stamped product 44.
Evaluation of Mechanical Properties
Vickers hardness of each of the first region (martensite), the third region (martensite and tempered martensite), and the second region (tempered martensite) 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 first region was 450 HV to 500 HV (1500 MPa to 1700 MPa), the third region was 270 HV to 450 HV (850 MPa to 1500 MPa), and the second region was 250 HV to 270 HV (800 MPa to 850 MPa). From the result, it was found that the hot stamped product having a hard portion (first region) excellent in strength, a soft portion (second region) which is easy to be post-processed, and the third region having strength therebetween was obtained in Example 3.
Measurement of Crystal Grain Size
The crystal grain size of martensite in the first region 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 11.8. From the result, it was found from the result that the crystal grain size of martensite in the first region was made finer by performing quenching twice from rapid short time heating in the same manner as in Example 1 also in Example 3.
This application claims priority to Japanese Patent Application No. 2018-196981 filed on October 18, 2018 and Japanese Patent Application No.2019-160716 filed on September 3, 2019, the entire contents of which are incorporated herein by reference.

Claims (6)

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

PCT/JP2019/040964 2018-10-18 2019-10-17 Hot stamping method and hot stamped product WO2020080486A1 (en)

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JP2019160716A JP7372787B2 (en) 2018-10-18 2019-09-03 Hot press forming method and hot press forming product

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Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102011101991B3 (en) * 2011-05-19 2012-08-23 Volkswagen Aktiengesellschaft Heat treatment of hardenable sheet metal components
WO2013137308A1 (en) 2012-03-13 2013-09-19 株式会社アステア Method for strengthening steel plate member
US20160001342A1 (en) * 2013-02-06 2016-01-07 Magna International Inc. Hot die forming assembly and method of makig a heat treated part
CN104668326B (en) * 2015-03-05 2016-08-24 山东大王金泰集团有限公司 A kind of hot stamping method of high strength steel parts capability gradientization distribution
JP2018079484A (en) 2016-11-14 2018-05-24 株式会社豊田中央研究所 Hot-press molding method and hot-press molded article
JP2018196981A (en) 2017-05-23 2018-12-13 三緯國際立體列印科技股▲ふん▼有限公司ＸＹＺｐｒｉｎｔｉｎｇ， Ｉｎｃ． Cleaning assembly for coloring head
JP2019160716A (en) 2018-03-16 2019-09-19 株式会社Ｇｓユアサ Power storage device

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102011101991B3 (en) * 2011-05-19 2012-08-23 Volkswagen Aktiengesellschaft Heat treatment of hardenable sheet metal components
WO2013137308A1 (en) 2012-03-13 2013-09-19 株式会社アステア Method for strengthening steel plate member
US20160001342A1 (en) * 2013-02-06 2016-01-07 Magna International Inc. Hot die forming assembly and method of makig a heat treated part
CN104668326B (en) * 2015-03-05 2016-08-24 山东大王金泰集团有限公司 A kind of hot stamping method of high strength steel parts capability gradientization distribution
JP2018079484A (en) 2016-11-14 2018-05-24 株式会社豊田中央研究所 Hot-press molding method and hot-press molded article
JP2018196981A (en) 2017-05-23 2018-12-13 三緯國際立體列印科技股▲ふん▼有限公司ＸＹＺｐｒｉｎｔｉｎｇ， Ｉｎｃ． Cleaning assembly for coloring head
JP2019160716A (en) 2018-03-16 2019-09-19 株式会社Ｇｓユアサ Power storage device

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