Hot stamping steel sheet, and hot stamping member with hot stamping steel sheet

Description

25 HOT STAMPING STEEL SHEET, AND HOT STAMPING MEMBER WITH HOT STAMPING STEEL SHEET TECHNICAL FIELD

[001] The present description refers to a hot stamping steel sheet and to a hot stamping member that uses the same. FUNDAMENTALS OF THE TECHNIQUE

[002] Conventionally, it is necessary to improve the safety of an occupant in a vehicle, and for this purpose, the shape of a component used in the vehicle body has been complicated to improve rigidity and the like. On the other hand, against the backdrop of the growing problem of global warming and the like, the movement to improve the fuel efficiency of automobiles is accelerating. It is known that reducing the weight of a vehicle body is effective in improving fuel efficiency. For this reason, automotive components need to achieve high strength and complex shapes. As a technique capable of achieving high strength and complex shapes, hot stamping has been proposed, in which a steel sheet is pressure-molded at high temperature using a mold and then rapidly cooled to provide the desired shape.

[003] In hot-stamped members produced by a hot stamping process, the tensile strength (TS) is primarily 1500 MPa or more. Hot-stamped members with a tensile strength of 1500 MPa or more are applied to non-deformable components that suppress cabin deformation in the event of an automobile collision. As the strength increases, the member's resistance to deformation increases, and the initial load at the moment of collision increases; thus, not only does the impact of the collision propagate to the cabin, but cracks are also easily generated and developed at the moment of collision. Petition 870250085607, dated 09 / 22 / 2025, p. 12 / 40 / 25 deformation, causing a problem whereby a sufficient collision energy absorption effect cannot be obtained.

[004] Therefore, it is known that the collision properties of the entire vehicle body are improved by the arrangement of a hot-stamped member with a tensile strength of less than 1500 MPa and an intensified energy absorption effect in a predetermined portion. In particular, in recent years, several hot-stamped components with tensile strength of less than 1500 MPa have been proposed as a hot-stamped component with tensile strength of 800 to 1300 MPa.

[005] Patent Document 1 describes a hot stamping member with high strength and capable of improving a hole expansion ratio correlated with impact properties by defining the metallic microstructure to 40% or more and 90% or less ferrite and 10% or more and 60% or less martensite.

[006] Patent Document 2 describes a hot stamping member that has high strength and high bending capacity by having a rigid inner layer, a soft surface layer phase obtained by decarburizing a portion of the surface and an inclined structure that constitutes a transition layer between the two layers in a thickness direction. DOCUMENT ON CONVENTIONAL TECHNIQUE PATENT DOCUMENT

[007] Patent Document 1: WO 2013 / 105633 A Patent Document 2: WO 2018 / 179839 A SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[008] Many portions of the metallic microstructure of the hot stamping member of Patent Document 1 are ferrite, and it is difficult Petition 870250085607, dated 09 / 22 / 2025, p. 13 / 40 / 25 defines tensile strength between 800 and 1300 MPa. 40% or more ferrite is present along with martensite, capable of improving tensile strength, causing a problem in that the stress due to deformation at the moment of collision is concentrated in the ferrite, cracks are easily generated and develop prematurely, and a sufficient collision energy absorption effect cannot be obtained.

[009] In the hot stamping member of the Document Patent 2 states that it is essential to provide a soft layer through decarburization. Decarburization is performed prior to hot stamping, so the decarburized state needs to be maintained during the hot stamping process. This creates a problem where process conditions, such as immersion temperature and time, cooling rate, and molding temperature to achieve a soft surface layer, are strictly limited. Furthermore, while it is possible to obtain a strength in the range of 800 to 1300 MPa, there is still the problem of not being able to achieve a sufficient collision energy absorption effect.

[0010] The present description was made in view of such a situation, and an objective of the present description is to provide a hot stamping steel sheet capable of obtaining a hot stamping member with an excellent collision energy absorption effect, and a hot stamping member with an excellent collision energy absorption effect. SOLUTIONS TO THE PROBLEMS

[0011] A first aspect of the present invention is a hot stamping steel sheet containing: C: 0.050 to 0.12% by mass; Si: 0.50 to 2.0% by mass; Mn: 2.1 to 3.0% by mass; P: 0.10% by mass or less (including 0% by mass); Petition 870250085607, dated 09 / 22 / 2025, p. 14 / 40 / 25 S: 0.010% by mass or less (including 0% by mass); Al: 0.01 to 0.10% by mass; Ti: 0.010 to 0.100% by mass; B: 0.0010 to 0.0100% by mass; N: 0.010% by mass or less (including 0% by mass); and a residual portion being Fe and unavoidable impurities, and satisfying the following formula (1). α = [%C] / 21 - [%Si] / 334 + [%Mn] / 81 - [%Ti] / 11 + 10 x <%B> > 0.03 ... (1)

[0012] Here, [%C], [%Si], [%Mn] and [%Ti] are the contents of C, Si, Mn and Ti represented in % by mass, respectively, and <%B> is the amount of solid solution B represented in % by mass.

[0013] A second aspect of the present invention is the hot stamping steel sheet according to the first aspect, wherein the hot stamping steel sheet is a hot-rolled steel sheet.

[0014] A third aspect of the present invention is the hot stamping steel sheet according to the first aspect, wherein the hot stamping steel sheet is a cold-rolled steel sheet.

[0015] The fourth aspect of the present invention is a hot stamping member with hot stamping steel sheet according to any one of the first to third aspects, wherein a metallic microstructure has a total martensite to bainite area ratio of 70% or more and a ferrite area ratio of 30% or less, and a tensile strength of 800 MPa or more and 1300 MPa or less. EFFECTS OF THE INVENTION

[0016] One embodiment of the present invention can provide a hot stamping steel sheet capable of providing a hot stamping member with an excellent energy absorption effect. Petition 870250085607, dated 09 / 22 / 2025, p. 15 / 40 / 25 collision, and another embodiment of the present invention can provide a hot stamping member with an excellent collision energy absorption effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] [Figure 1] Figure 1 shows a sectional view of a hot stamping member according to an embodiment.

[0018] [Figure 2] Figure 2 shows a schematic view of a crush test specimen. DETAILED DESCRIPTION

[0019] The present inventors conducted intensive studies to solve the above problems. In particular, as a result of detailed studies focusing on the microstructure after hot stamping, it was found that it is necessary to suppress the amount of ferrite in the metal microstructure in the state after hot stamping, i.e., in the state of the hot stamping member. More specifically, it was found that not only is the content of each of C, Si, Mn, P, S, Al, Ti, B and N defined within a predetermined range, but also the following formula (1) is satisfied, thus allowing the metallic microstructure of the hot stamping member to have a total area ratio of martensite and bainite of 70% or more and ferrite of 30% or less and a tensile strength of 800 to 1300 MPa and, as a result, an excellent collision energy absorption effect can be obtained. α = [%C] / 21 - [%Si] / 334 + [%Mn] / 81 - [%Ti] / 11 + 10 x <%B> > 0.03 ... (1)

[0020] Here, [%C], [%Si], [%Mn] and [%Ti] are the contents of C, Si, Mn and Ti represented in % by mass, respectively, and <%B> is the amount of solid solution B represented in % by mass.

[0021] When using the steel plate according to the embodiment of the present invention, in which the content of each of C, Si, Mn, P, S, Al, Ti, B Petition 870250085607, dated 22 / 09 / 2025, p. 16 / 40 / 25 and N is adjusted to be within a predetermined range and the value of α determined by formula (1) is adjusted to be within a predetermined range, a hot stamping material obtained when special hot stamping conditions are not applied, but general hot stamping conditions, can have an excellent collision energy absorption effect.

[0022] Next, each requirement defined in the embodiments of the present invention will be described in detail. 1. Chemical composition

[0023] The steel sheet and the hot stamping member according to the embodiment of the present invention can have an excellent collision energy absorption effect after hot stamping (i.e., the state of the hot stamping member) by having the chemical composition described below. [C: 0.050% by mass or more and 0.12% by mass or less]

[0024] In the present invention, C is an important element. To suppress excessive strength increase, ensuring a certain degree of hardenability and suppressing ferrite generation, it is necessary to define the amount of C within an appropriate range. Excessively reducing the amount of C does not guarantee the strength and hardenability of the hot-stamped member, and therefore the amount of C is defined as 0.050% by mass or more. The amount of C is preferably 0.060% by mass or more, and more preferably 0.070% by mass or more. In contrast, an excessive amount of C leads to excessive strength after hot stamping, and the toughness of the hot-stamped member is insufficient, thus causing cracks due to deformation at the moment of collision, which can easily occur and develop, and the collision energy absorption properties can be deteriorated, and thus the upper limit of the amount of C is defined as 0.12% by mass. The amount of C is... Petition 870250085607, dated 09 / 22 / 2025, page 17 / 40 / 25 preferably 0.11% by mass or less, and more preferably 0.10% by mass or less. [Si: 0.50% by mass or more and 2.0% by mass or less]

[0025] Silicon (Si) is an important element for ensuring the strength of hot-stamped members. The addition of Si suppresses the precipitation of C as a carbide during cooling after hot stamping, thus ensuring strength. To exhibit such an effect, the amount of Si is defined as 0.50% by mass or more. The amount of Si is preferably 0.60% by mass or more, and more preferably 0.70% by mass or more. However, an excessive amount of Si causes deterioration of pickling properties due to increased oxide fouling during the production of a steel sheet, an increase in the Ac3 transformation temperature, a decrease in the cross-tensile strength of a spot-welded portion, and the like. Therefore, the amount of Si is 2.0% by mass or less. The amount of Si is preferably 1.8% by mass or less, and more preferably 1.6% by mass or less. [Mn: 2.1% by mass or more and 3.0% by mass or less]

[0026] In the present invention, Mn is an important element. It is useful for improving the hardenability of steel sheet and making the main phase of the metallic microstructure after hot stamping martensite and bainite, while the amount of C is adjusted to the above range. To exhibit such an effect, the amount of Mn is 2.1% by mass or more. However, when the amount of Mn exceeds 3.0% by mass and is excessive, the effect is saturated, which not only causes an increase in cost but also leads to an excessive increase in the strength of the hot-rolled sheet and a decrease in cold rolling later in the production of the steel sheet before hot stamping. Therefore, the amount of Mn is 3.0% by mass or less. The amount of Mn is preferably 2.8% by mass or Petition 870250085607, dated 09 / 22 / 2025, page 18 / 40 / 25 less, and more preferably 2.6% by mass or less. [P: 0.10% by mass or less (including 0% by mass);

[0027] P is an unavoidable impurity element and is an element that deteriorates the weldability of steel plate and, therefore, it is desirable to limit its content as much as possible. In order not to deteriorate the weldability of the steel plate, the amount of P is defined as 0.10% by mass or less. The amount of P is preferably 0.050% by mass or less, and more preferably 0.020% by mass or less. P is an impurity inevitably mixed into steel, and it is difficult to define its quantity as 0% by mass in industrial production, and an amount of 0.0005% by mass or more is normally present.

[0028] In this descriptive report, “including 0% by mass” means including an embodiment in which the elements are not intentionally added, i.e., a case in which the content is equal to or less than the unavoidable impurity level (the case in which the elements are intentionally added is not excluded). [S: 0.010% by mass or less (including 0% by mass)]

[0029] S is an unavoidable impurity element and deteriorates the weldability of the steel plate. Therefore, the amount of S is 0.010% by mass or less. The amount of S is preferably 0.0080% by mass or less, and more preferably 0.0050% by mass or less. The amount of S is preferably as small as possible, but it is difficult to define the amount of S as 0% by mass in industrial production, and an amount of 0.0001% by mass or more is normally contained. [Al: 0.01% by mass or more and 0.10% by mass or less]

[0030] Al is an element that acts as a deoxidizer. To exhibit such an effect, the amount of Al is defined as 0.01% by mass or more. The amount of Al is preferably 0.015% by mass or more, and more preferably 0.020% by mass or more. However, excessive Al content Petition 870250085607, dated 09 / 22 / 2025, page 19 / 40 / 25 leads to an increase in the cost of production and, therefore, the quantity of Al is defined as 0.10% by mass or less. The quantity of Al is preferably 0.08% by mass or less, and more preferably 0.07% by mass or less. [Ti: 0.010% by mass or more and 0.100% by mass or less]

[0031] Ti is an important element in the present invention. The presence of Ti promotes the formation of TiN, thus increasing the amount of solid solution B, which improves hardenability (it is possible to suppress the N-B bond). To exhibit such an effect, the amount of Ti is 0.010% by mass or more. The amount of Ti is preferably 0.012% by mass or more, and more preferably 0.015% by mass or more. However, an excessive amount of Ti causes TiC to be easily generated, the steel structure to be refined, and hardenability to be significantly deteriorated. Therefore, the amount of Ti is 0.100% by mass or less, preferably 0.095% by mass or less, and more preferably 0.090% by mass or less. [B: 0.0010% by mass or more and 0.0100% by mass or less]

[0032] In the present invention, B is an important element. B is known as a grain boundary strengthening element and is contained as an effective element to improve hardenability. To obtain such an effect, the amount of B needs to be 0.0010% by mass or more. The amount of B is preferably 0.0012% by mass or more, and more preferably 0.0015% by mass or more. However, an excessive amount of B becomes a factor causing surface cracking or similar defects during casting due to the generation of compound B or similar. Therefore, the amount of B needs to be 0.0100% by mass or less, preferably 0.0090% by mass or less, and more preferably 0.0085% by mass or less. [N: 0.010% by mass or less (including 0% by mass)] Petition 870250085607, dated 09 / 22 / 2025, p. 20 / 40 / 25

[0033] N is an unavoidable impurity element and, when N is excessively contained, BN is readily generated and the amount of solid solution B is reduced. Therefore, the amount of N is 0.010% by mass or less. The amount of N is preferably 0.008% by mass or less, and more preferably 0.005% by mass or less. The amount of N is preferably as small as possible and therefore the lower limit is not particularly restricted, but it is difficult to define the amount as 0% by mass in industrial production, and an amount of 0.0001% by mass or more is normally contained. [Residual portion]

[0034] The basic components of the hot-stamping steel sheet and the hot-stamping member according to the embodiment of the present invention are as described above, and the residual portion is iron and unavoidable impurities. Examples of unavoidable impurities include, in addition to O, which is preferably suppressed as described below, and residual elements (Pb, Bi, Sb, Sn, V and the like) brought in, depending on the situations of raw materials, materials, production facilities and the like, within a range in which the effects of the present invention are not impaired.

[0035] There are elements such as P, S and N, which are normally preferred because the content is lower and therefore they are unavoidable impurities, but they are defined separately according to the composition range described above. Therefore, in this descriptive report, the “unavoidable impurities” that constitute the residual portion indicate a concept that excludes elements with a separately defined composition range.

[0036] Oxygen is an unavoidable impurity element and, when included in excess, forms an oxide and the solid Si solution is reduced. Therefore, the amount of oxygen is preferably 0.010% by mass or less. The amount of oxygen is more preferably 0.005% by mass or less, and still Petition 870250085607, dated 22 / 09 / 2025, p. 21 / 40 / 25 more preferably 0.003% by mass or less. The quantity of O is preferably as small as possible and therefore the lower limit is not particularly restrictive, but it is difficult to define the quantity as 0% by mass in industrial production, and a quantity of 0.0001% by mass or more is normally contained. [Value α of formula (1)]

[0037] To define the metallic microstructure of the hot stamping member obtained when general hot stamping conditions are applied to the hot stamping steel sheet of the present invention for 70% or more of total martensite and bainite and 30% or less of ferrite in terms of area ratio, it is necessary not only to define the content of each of C, Si, Mn, P, S, Al, Ti, B and N within the range predetermined above, but also to satisfy the following formula (1). α = [%C] / 21 - [%Si] / 334 + [%Mn] / 81 - [%Ti] / 11 + 10 x <%B> > 0.03 ... (1)

[0038] Here, [%C], [%Si], [%Mn] and [%Ti] are the contents of C, Si, Mn and Ti represented in % by mass, respectively, and <%B> is the amount of solid solution B represented in % by mass.

[0039] The α value obtained by formula (1) above is an index that indicates the hardenability of the steel sheet. Ferrite fractions were measured from several samples obtained by applying general hot stamping conditions, multiple regression analysis was performed with the ferrite fraction as an objective variable and each component value as an explanatory variable, and the following formula (2) was obtained from the relational formula between the ferrite fraction and the component. α = [%C] / 21 - [%Si] / 334 + [%Mn] / 81 - [%Ti] / 11 + 10 x <%B> - [%Nb] / 2 > 0.03 ... (2)

[0040] Here, [%C], [%Si], [%Mn], [%Ti] and [%Nb] are the contents of C, Si, Mn and Ti and Nb plotted in % by mass, respectively, and <%B> is the amount of solid solution B plotted in % by mass. Petition 870250085607, dated 09 / 22 / 2025, p. 22 / 40 / 25

[0041] Given that the denominator of the coefficient of the Nb term is small and it is difficult to guarantee α > 0.03 in a useful quantity of Nb added, Nb is not contained in formula (2) (i.e., without addition of Nb), thus reaching formula (1).

[0042] The quantity of solid solution B, <%B>, can be determined as follows.

[0043] NO present in steel binds more readily to Ti than to B. Under the condition that all the N is bound to Ti to form TiN, that is, when the following formula (3) is satisfied due to the atomic weights 47.8 and 14 of Ti and N, respectively, and since TiN is 1:1 in stoichiometric ratio, all the N are converted to TiN, BN is not formed and all the B added (amount of B) is determined as a solid solution B. That is, the amount of solid solution B, <%B>, is equal to the amount of B. [%Ti] > (47.8 / 14) x [%N] ... (3)

[0044] Here, [%Ti] and [%N] are the Ti and N contents represented in % by mass, respectively.

[0045] In contrast, when formula (3) above is not satisfied, the N that did not form TiN forms BN and therefore the amount of solid solution B, <%B>, is determined using the following formula (4), also using the atomic weight of B of 10.8. However, if the value of formula (4) is negative, <%B> is defined as zero. <%B> = 10.8 x ([%B] / 10.8 - ([%N] / 14 - [%Ti] / 47.8)) ... (4)

[0046] Here, [%B], [%Ti] and [%N] are the contents of B, Ti and N represented in % by mass, respectively.

[0047] The “general hot stamping conditions” described above are conditions that satisfy (1) heating to an immersion temperature of Ac3 or higher and holding at the immersion temperature (immersion), (2) an average cooling rate from the temperature of Petition 870250085607, dated 22 / 09 / 2025, p. 23 / 40 / 25 immersion to the molding temperature, the cooling rate being equal to or greater than the natural cooling rate, and (3) a molding temperature of 500 to 750°C. “Molding temperature” means the temperature of the steel sheet immediately before molding with a mold, and “equal to or greater than the natural cooling rate” means a cooling rate when the steel sheet is removed from the heating furnace and then cooled naturally in air without heating to decrease the cooling rate. 2. Metallic microstructure (1) Metallic microstructure of hot stamping steel sheet

[0048] The hot stamping steel sheet according to the embodiment of the present invention is a steel sheet that can be processed into a desired shape by applying the “general hot stamping conditions” described above. Heating is carried out at a temperature of Ac3 or higher during hot stamping and therefore there is no specific restriction on the microstructure of the metal in the steel sheet state. The hot stamping steel sheet according to the embodiment of the present invention can be a hot-rolled steel sheet or a cold-rolled steel sheet.

[0049] Hot-rolled steel sheet does not need to be subjected to cold rolling after hot rolling and offers excellent productivity. In contrast, cold-rolled steel sheet has the advantage of easily being able to obtain a thin steel sheet (2) Hot stamping member

[0050] The hot stamping member according to the embodiment of the present invention can be obtained by applying “general hot stamping conditions” to the steel sheet described above for hot stamping according to the embodiment of the present invention.

[0051] The metallic microstructure of the stamping member a Petition 870250085607, dated 09 / 22 / 2025, p. 24 / 40 / 25 The hot-melted material obtained has 70% or more of the total martensite and bainite, 30% or less ferrite, and less than 1% of other microstructures by area ratio. Examples of other microstructures include pearlite and retained austenite. Preferably, other microstructures are not included. That is, preferably, the metallic microstructure includes only 70% or more of the total martensite and bainite and 30% or less ferrite by area ratio.

[0052] 70% or more of the total martensite and bainite in an area ratio allows the tensile strength of the resulting hot-stamped member to be 800 MPa or more and 1300 MPa or less. This makes it possible to ensure a sufficient amount of energy absorption at the moment of impact. The total martensite and bainite is preferably 80% or more, more preferably 90% or more in an area ratio.

[0053] Both martensite and bainite may be included, or only one of them may be included. The martensite may be one or more selected from the group consisting of carbide-quenched martensite, carbide-quenched martensite, and self-quenched martensite. The bainite may be one or both upper bainites, including cementite and austenite retained between the lamellae, and lower bainites, including carbides within the lamellae.

[0054] Ferrite is an equiaxed crystal grain that does not contain iron-based carbide. Setting the ferrite area ratio at 30% or less makes it possible to reduce a structure in which the stress due to deformation at the moment of collision is concentrated and to suppress the occurrence and development of cracks at an early stage. The ferrite area ratio is preferably 20% or less, and more preferably 10% or less.

[0055] The metallic microstructure is observed at a sheet thickness position t / 4 (a position at a distance of 1 / 4 of the sheet thickness t from the surface (main surface) of the steel sheet that constitutes the Petition 870250085607, dated 09 / 22 / 2025, page 25 / 40 / 25 hot stamping member towards the center), which is a representative portion of the flat portion (portion that does not exhibit hot deformation, such as bending) of the hot stamping member.

[0056] The surfaces of the hot-stamping steel sheet and the hot-stamping member according to the embodiment of the present invention may or may not have a coated layer, such as a hot-dip galvanized coating or a zinc-alloyed coating. Having such a coated layer, for example, allows suppressing the generation of fouling during hot stamping heating. In contrast, when the coated layer is not provided, the production cost of the steel sheet can be reduced. Furthermore, the hot-stamping steel sheet and the hot-stamping member according to the embodiment of the present invention may have a decarburized layer on the surface to improve formability during hot stamping. 3. Production method

[0057] In the hot-stamped steel sheet according to the embodiment of the present invention, the metallic microstructure is not particularly limited, as described above. Therefore, provided the chemical composition defined in item “1” above is met, if the “chemical composition”, including formula (1), is satisfied, the production of hot-rolled steel sheet and cold-rolled steel sheet can be easily carried out using a known method.

[0058] The hot stamping steel sheet according to the embodiment of the present invention can be obtained by performing hot stamping under conditions that satisfy the “general hot stamping conditions” described above: (1) heating to an immersion temperature of Ac3 or higher and holding at the immersion temperature (immersion); (2) an average cooling rate from the immersion temperature to Petition 870250085607, dated 09 / 22 / 2025, p. 26 / 40 / 25 a forming temperature, being greater than a cooling rate by natural cooling; and (3) a forming temperature of 500 to 750°C.

[0059] Examples of immersion temperatures of Ac3 or higher include temperatures of 850 to 1000°C. Additionally, examples of immersion times (retention times) to retain these immersion temperatures include 50 to 300 seconds.

[0060] In addition, examples of “more than a natural cooling rate”, which is an average cooling rate from the immersion temperature to the molding temperature, include natural cooling (e.g., 2°C / s) at 50°C / s. EXAMPLE 1. Example 1

[0061] In Example 1, the relationship between the value of α obtained by composition and formula (1) was clarified, and the metallic microstructure of the sample after hot stamping (hot stamping member). (1) Preparation of sheet steel sample for hot stamping

[0062] Steel with the components shown in Table 1 was melted to form a plate. In addition to the chemical composition, Table 1 also shows the value α calculated by formula (1). For steel types A, D, E, and I containing B, the value on the right-hand side of formula (3) and whether formula (3) is satisfied or not were described. As verified in Table 1, all steel types A, D, E, and I satisfy formula (3) and therefore the quantity B of each steel type was used as the value of the quantity B of solid solution. , in the calculation α using formula (1).

[0063] The resulting plate was heated to a temperature of 1100 to 1300°C, hot-rolled so that the final rolling temperature was 890 to 950°C, and held between 500 and 700°C. After the holding temperature, cooling to room temperature was carried out to obtain a Petition 870250085607, dated 09 / 22 / 2025, page 27 / 40 / 25 hot-rolled steel sheet. The hot-rolled steel sheet obtained was pickled to remove surface fouling and then cold-rolled to produce a cold-rolled steel sheet. The thickness of the cold-rolled steel sheet thus obtained is shown in Table 2. For the cold-rolled steel sheet sample, the cold-rolled steel sheet was subjected to additional annealing as needed, and hot-dip galvanizing or alloy hot-dip galvanizing treatment was performed. [Table 1] Chemical composition (% by mass), residual portion: Fe and unavoidable impurities α Value on the right-hand side of formula (3) Satisfaction / dissatisfaction of formula (3) Steel type C Si Mn PS Al Ti B Nb NA 0.08 5 1.18 2.2 3 0.0 11 0.00 2 0.0 36 0.0 25 0.00 16 0 0.00 31 0.04 2 0.0106 Satisfaction B 0.05 3 1.24 *0.21 0.0 10 0.00 2 0.0 35 *0 *0 0 0.00 31 *0.0 014 ^- ^- C *0.0 46 1.21 *0.21 0.0 10 0.00 1 *0. 487 *0 *0 0 0.00 40 *0.0 012 ^- ^- D 0.07 7 *0.3 5 *1.59 0.0 10 0.00 2 0.0 36 0.0 22 0.00 22 *0.0 50 0.00 27 *0.0 17 0.0092 Satisfaction E 0.06 0 *0.0 2 *1.54 0.0 09 0.00 2 0.0 35 0.0 71 0.00 15 *0.0 51 0.00 27 *0.0 049 0.0092 Satisfaction F 0.06 9 *0 2.1 0 0.0 10 0.00 3 0.0 45 *0 *0 *0.0 50 0.00 20 *0.0 042 ^- ^- G 0.08 0 *0.0 3 *1.45 0.0 17 0.00 1 0.0 35 *0 *0 *0.0 32 0.00 20 *0.0 056 ^- ^- H 0.08 2 1.13 2.2 7 0.0 10 0.00 1 0.0 44 0.0 29 0.00 19 0 0.00 49 0.04 5 0.0167 Satisfaction I *0.2 20 1.15 2.2 3 0.0 10 0.00 1 0.0 40 0.0 31 0.00 20 0 0.00 25 0.05 2 0.0085 Satisfaction * means outside the scope defined by the embodiments of the present invention.

[0064] For the steel sheet sample thus obtained, a blank was cut with the size shown in Table 2, and hot stamping was performed on the resulting blank under the conditions shown in Table 2 (immersion temperature, immersion time, average cooling rate from immersion temperature to molding temperature, and molding temperature) for Petition 870250085607, dated 09 / 22 / 2025, page 28 / 40 / 25 to obtain a sample of a hot stamping member. [Table 2] Steel Type Sample Name Sheet Thickness [millimeters] Sketch Size [millimeters] Immersion Temperature [°C] Immersion Time [s] Average Cooling Rate from Immersion Temperature to Forming Temperature [°C / s] Forming Temperature [°C] Ferrite Fraction after Hot Stamping [% by Area] A A1 1.4 150x25 0 890 200 30 550 0 A2 1.4 180x70 900 100 9.5 700 0 B B1 1.4 150x25 0 890 200 30 550 *97 B2 1.4 180x70 900 100 9.5 700 *100 C C1 1.4 150x25 0 890 200 30 550 *100 C2 1.4 180x70 900 100 9.5 700 *96 D D1 1.4 150x25 0 890 200 30 550 *56 D2 1.4 180x70 900 100 9.5 700 9 E E1 1.4 150x25 0 890 200 30 550 *88 E2 1.4 180x70 900 100 9.5 700 *71 F F1 1.2 150x25 0 890 300 20 750 *90 F2 1.2 150x25 0 890 300 Natural cooling 750 *83 G G1 1.4 150x25 0 890 300 20 750 *88 G2 1.4 150x25 0 890 300 Natural cooling 750 *86 H H1 1.2 217x30 0 930 180 12.3 550 0 H2 1.4 150x25 0 890 200 30 550 0 I I1 1.2 217x30 0 930 180 12.3 550 0 I2 1.4 150x25 0 890 200 30 550 0 * means that the ferrite fraction exceeds 30% by area ratio.

[0065] Results of microstructure observation and ferrite fraction measurement of the hot stamping member sample obtained Petition 870250085607, dated 09 / 22 / 2025, pages 29 / 40 / 25 are shown in Table 2.

[0066] The microstructure observation was performed by observing a region of 100 μm in length x 130 pm in width in a visual field in a sheet thickness at the t / 4 position on a surface parallel to the rolling direction, which had been chemically etched with nital, using a scanning electron microscope (SEM) at a magnification of 1000 to 3000.

[0067] No microstructure other than ferrite, martensite, and bainite was observed in any of the samples. That is, microstructures other than ferrite were all martensite or bainite.

[0068] The ferrite fraction was determined by a point counting method in which grids at 100 or more intersections of vertical and horizontal lines were uniformly arranged in a SEM image 1,000 to 3,000 times subjected to microstructure observation, and the number of intersections in each phase was counted.

[0069] The chemical compositions of samples A1, A2, H1 and H2 satisfy the defined component range, and the ferrite fraction after hot stamping performed under common hot stamping conditions is 0% (100% for martensite and bainite in total), also satisfying the α value.

[0070] In samples B1, B2, C1, C2, D1, E1, E2, F1, F2, G1 and G2, the chemical composition does not satisfy the definition, the value of α is not satisfied and the ferrite fraction is greater than 30%.

[0071] In sample D-2, Si and Mn are lower than the specified values. Also due to this influence, the ferrite fraction was 30% or less, although the α value deviated. However, the Si value is low and therefore it is considered that it is not possible to obtain sufficient strength.

[0072] Samples I1 and I2 showed an α value within the Petition 870250085607, dated 09 / 22 / 2025, page 30 / 40 / 25 defined range and a ferrite fraction equal to 0. However, the amount of C is excessively large, the resistance is excessively high, and it is not possible to obtain sufficient collision energy absorption properties (see also Example 2). 2. Example 2

[0073] In Example 2, in addition to Samples H1 and I1 from Example 1, three types of general-purpose high-strength steel sheets (cold-rolled steel sheets) for cold forming (steel types J, K, and L) with the chemical compositions shown in Table 3 as comparative materials were evaluated for collision energy absorption properties. [Table 3] Chemical composition (% by mass), residual portion: Fe and unavoidable impurities Steel type C Si Mn PS Al Ti B Nb J 0.07 0 *0 2.05 0.01 8 0.00 3 0.04 5 *0 *0 0 High-strength steel sheet for cold forming K 0.12 3 *0.1 0 2.67 0.01 0 0.00 2 0.04 5 0.07 0 *0 0 High-strength steel sheet for cold forming L *0.1 70 1.35 2.10 0.01 5 0.00 1 0.04 0 0.03 0 *0 0 High-strength steel sheet for cold forming * means outside the scope defined by the embodiments of the present invention. (1) Sample preparation for crush test

[0074] Sketch samples for samples H1-1 and H1-2 having a width of 217 mm x a length of 300 mm x a thickness of 1.2 mm, as in Example 1, were prepared in the same manner as in H1 of Example 1. As shown in Table 4, the sketch for Sample H1-1 and the sketch for Sample H1-2 are the same, except that the former is not subjected to coating and the latter is subjected to alloyed zinc coating.

[0075] Sketch samples for samples I1-1 and I1-2 with a width of 217 mm x a length of 300 mm x a thickness of 1.2 Petition 870250085607, dated 09 / 22 / 2025, page 31 / 40 / 25 mm were prepared as in Example 1 in the same manner as in I1 of Example 1. As shown in Table 4, the sketch for Sample I1-1 and the sketch for Sample I1-2 are the same, except that the former is subjected to an alloyed zinc coating, and the latter has a decarburized surface layer in addition to the alloyed zinc coating.

[0076] Using these samples from the sketch, a hot stamping member sample with the cross-sectional shape of Figure 1 was prepared as in Example 1. The hot stamping conditions for Samples H1-1 and H1-2 are the same as for Sample H1 in Example 1, and the hot stamping conditions for Samples I1-1 and I1-2 are the same as for Sample I1 in Example 1.

[0077] In addition, cold forming samples of samples J-1, K-1 and L-1 having a width of 217 mm x a length of 300 mm x a thickness of 1.2 mm were cut from cold forming samples of high-strength steel sheets of steel types J, K and L, respectively. The cold forming sample obtained was cold to obtain a cold-formed sample of the high-strength steel sheet with the cross-section of Figure 1.

[0078] Next, both ends of the hot-stamped member obtained and the cold-formed sample of high-strength steel sheet (the hot-stamped member and the cold-formed sample of high-strength steel sheet can be collectively referred to as the "hat member") were cut to a length of 200 mm. In addition, a 590 Ma grade steel sheet of the same thickness was attached to a flange portion of a hat member having a length of 200 mm by spot welding to form a closed sectional structure.

[0079] In addition, a top plate was attached to a portion of the upper end of the hat member by arc welding, and a plate Petition 870250085607, dated 09 / 22 / 2025, page 32 / 40 / 25. The lower plate was attached to a portion of the lower end of the hat member by arc welding. The upper plate was 200 mm x 200 mm, the lower plate was 250 mm x 250 mm, and the plate thickness was 9 mm in both cases. Both the upper and lower plates were positioned so that the center of gravity coincided with the hat member.

[0080] Figure 2 shows a schematic view of a crush test specimen obtained in this way. In Figure 2, the plate fixed to the flange portion of the cap member by spot welding is omitted so that the shape of the cap member can be easily recognized, but the spot welding positions provided at 25 mm intervals are shown. (2) Crush test

[0081] Using the obtained crush test specimen, a dynamic axial crush test was performed, in which a falling weight of 190 kg was dropped from a height of 10 m. A stop to receive the falling weight was placed at a position 105 mm downwards from the top plate of the crush test specimen. The load was measured using a load cell, and the displacement was marked on the falling weight and measured with a laser displacement gauge.

[0082] A point where the falling weight was in contact with the crushing specimen and the load was recorded in the load cell was defined as a 0% stroke, and a point where the displacement was 100 mm was defined as a 100% stroke. The recorded data were calculated using a 21-point moving average.

[0083] The load on courses from 0 to 100% and the load on courses from 80 to 100% were integrated to calculate the amount of energy absorbed from each load.

[0084] The value obtained by dividing the amount of energy absorbed Petition 870250085607, dated 09 / 22 / 2025, page 33 / 40 / 25 in courses from 80 to 100% by the amount of energy absorbed in courses from 0 to 100% was represented in % to provide the EAR. The test was performed at least three times on each sample, and the average value of the same was shown in Table 4 as a result of the measurement of each sample. Samples with a total amount of absorbed energy of 5.5 kJ or more and an EAR of 18% or more were determined to have high absorbed energy.

[0085] In addition, the crushing state of each sample was observed and shown in Table 4. A sample in which a large crack occurred and led to fracture once in a plurality of tests was described as "large crack occurred and led to fracture" in Table 4, and a sample in which all samples were deformed into a bellows shape that was a normal form of crushing was described as "bellows deformation" in Table 4.

[0086] Samples having a total amount of absorbed energy of 5.5 kJ or more, an EAr of 18% or more, and a crushing form of all samples being bellows deformation were determined to have excellent collision energy absorption properties. Petition 870250085607, dated 09 / 22 / 2025, page 34 / 40 / 25 [Table 41] Crushing method Bellows deformation Bellows deformation Large crack occurred and led to fracture Large crack occurred and led to fracture Bellows deformation Large crack occurred and led to fracture Large crack occurred and led to fracture EAr [%1 18.4 19.0 14.6 15.2 19.9 14.2 17.3 Total amount of energy absorbed [kJ] 7.93 7.08 6.48 C4 4.78 6.36 7.85 Amount of energy absorbed in strokes from 80 to 100% [kJ1 1.46 1.35 0.95 0.95 0.90 1.36 Tensile strength [MPa] 1168 1124 *1623 *1553 *617 1036 1251 Plate thickness [millimeters] 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Surface property No plating Bonded galvanizing Bonded galvanizing Bonded galvanizingSurface decarburization Bonded galvanizing Bonded galvanizing No plating Ferrite fraction [%1 OOOO *86 *62 *32 Sample shape High-strength steel sheet for cold forming Hot stamping Hot stamping Hot stamping Hot stamping * High-strength steel sheet for cold forming * High-strength steel sheet for cold forming * High-strength steel sheet for cold forming Sample name H1-1 H1-2 ΓΠ I1-2 ΓΓ K-1 L-1 Steel type KM ►2, * means outside the scope defined by the embodiments of the present invention. of 22 / 09 / 2025, page 35 / 40 / 25 (Resistance)

[0087] A test specimen no. 5 (JIS test specimen no. 5) defined by JIS Z 2241 was taken from a flat portion (portion without hot deformation, such as bending) of the cap member of each sample, and the tensile strength was measured. The measurement results are shown in Table 4.

[0088] Samples H1-1 and H1-2 according to embodiments of the present invention exhibited excellent collision energy absorption properties.

[0089] The members of samples I1-1 and I1-2 exhibited excessively high tensile strength, had a low EAR in the crush test, and fractured due to the generation of large cracks.

[0090] Sample J-1 showed low tensile strength and low total amount of energy absorbed.

[0091] Samples K-1 and L-1 showed low EAR in the crush and fracture test due to the generation of large cracks.

[0092] This application claims priority based on a Japanese patent application, JP-2023-062285, filed on April 6, 2023. JP2023-062285 is incorporated herein by reference. Petition 870250085607, dated 09 / 22 / 2025, pp. 36 / 40

Claims

1 / 2 CLAIMS 1. Hot stamping steel sheet, characterized by the suit comprising: C: 0.050 to 0.12% by mass; Si: 0.50 to 2.0% by mass; Mn: 2.1 to 3.0% by mass; P: 0.10% by mass or less (including 0% by mass); S: 0.010% by mass or less (including 0% by mass); Al: 0.01 to 0.10% by mass; Ti: 0.010 to 0.100% by mass; B: 0.0010 to 0.0100% by mass; N: 0.010% by mass or less (including 0% by mass); and residual portion being Fe and unavoidable impurities, and satisfying the following formula (1): α= [%C] / 21 - [%Si] / 334 + [%Mn] / 81 - [%Ti] / 11 + 10 x <%B> > 0.03 ... (1) where [%C], [%Si], [%Mn] and [%Ti] are contents of C, Si, Mn and Ti represented in % by mass, respectively, and <%B> is an amount of solid solution B represented in % by mass.

2. Hot stamping steel sheet according to claim 1, characterized in that the hot stamping steel sheet is a hot-rolled steel sheet.

3. Hot stamping steel sheet according to claim 1, characterized in that the hot stamping steel sheet is a cold-rolled steel sheet.

4. Hot stamping member with hot stamping steel sheet as defined in any one of claims 1 to 3, characterized in that a metallic microstructure has a total martensite to bainite area ratio of 70% or more and a ferrite area ratio of 30% or less, and a tensile strength of 800 MPa or more and 1300 MPa or less.

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