Steel sheet for hot stamping and hot stamp member using same

Abstract

A steel sheet for hot stamping, including: C: 0.050 to 0.12 mass%; Si: 0.50 to 2.0 mass%; Mn: 2.1 to 3.0 mass%; P: 0.10 mass% or less (including 0 mass%); S: 0.010 mass% or less (including 0 mass%); Al: 0.01 to 0.10 mass%; Ti: 0.010 to 0.100 mass%; B: 0.0010 to 0.0100 mass%; and N: 0.010 mass% or less (including 0 mass%), with the residual portion being Fe and inevitable impurities, and satisfying the following formula (1): α = [%C] / 21 - [%Si] / 334 + [%Mn] / 81 - [%Ti] / 11 + 10 × <%B> ≥ 0.03 ... (1) Herein, [%C], [%Si], [%Mn], and [%Ti] are the contents of C, Si, Mn, and Ti represented in mass%, respectively, and <%B> is the amount of solid solution B represented in mass%.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel sheet for hot stamping and a hot stamp member using the same.BACKGROUND ART

[0002] Conventionally, it has been required to improve safety of an occupant in a vehicle, and for this purpose, a shape of a component used in a vehicle body has been made complicated to improve rigidity and the like. On the other hand, against the background of the growing global warming problem and the like, the movement for improving the fuel efficiency of automobiles is accelerating. It is known that weight reduction of a vehicle body is effective for improving fuel efficiency. For this reason, automobile components are required to achieve both high strength and complicated shapes. As a technique capable of achieving both high strength and complicated shape, hot stamping has been proposed in which a steel sheet is press-molded at a high temperature using a mold and then rapidly cooled to provide a desired shape.

[0003] In the hot stamp member produced by a hot stamping process, the tensile strength (TS) is mainly 1500 MPa or more. The hot stamp member having a tensile strength of 1500 MPa or more is applied to a non-deformable component that suppresses deformation of the cabin in the event of a collision of an automobile. As the strength increases, the deformation resistance of the member increases, and the initial load at the time of collision increases, and thus not only impact of collision propagates to the cabin, but also cracks are easily generated and developed at the time of deformation, causing a problem that a sufficient collision energy absorbing effect cannot be obtained.

[0004] Therefore, it is known that collision properties of the entire vehicle body are improved by arranging a hot stamp member having a tensile strength of less than 1500 MPa and an enhanced energy absorption effect at a predetermined portion. In particular, in recent years, various hot stamping components having a tensile strength of less than 1500 MPa, such as a hot stamping component having a tensile strength of 800 to 1300 MPa, have been proposed.

[0005] Patent Document 1 discloses a hot stamp member having high strength and capable of improving a hole expansion ratio correlated with impact properties by setting the metal microstructure to 40% or more and 90% or less of ferrite and 10% or more and 60% or less of martensite.

[0006] Patent Document 2 discloses a hot stamp member having both high strength and high bendability by having an inner hard layer, a surface layer soft phase obtained by decarburizing a surface portion, and an inclined structure constituting a transition layer between the two layers in a thickness direction.CONVENTIONAL ART DOCUMENTPATENT DOCUMENT

[0007] Patent Document 1: WO 2013 / 105633 A Patent Document 2: WO 2018 / 179839 A SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0008] Many portions of the metal microstructure of the hot stamp member of Patent Document 1 are ferrite, and it is difficult to set the tensile strength to 800 to 1300 MPa. 40% or more of ferrite is present together with martensite capable of improving the tensile strength, causing a problem that strain due to deformation at the time of collision concentrates on ferrite, cracks are easily generated and developed early, and a sufficient collision energy absorbing effect cannot be obtained.

[0009] In the hot stamp member of Patent Document 2, it is essential to provide a soft layer by decarburization. Decarburization is performed previously before hot stamping, whereby the decarburized state needs to be maintained in the hot stamping process, thus causing a problem that process conditions such as a soaking temperature and a soaking time, and a cooling rate and a molding temperature until molding for forming a surface soft layer are strictly limited. In addition, although the strength in the range of 800 to 1300 MPa can be obtained, there is still a problem that a sufficient collision energy absorbing effect cannot be obtained.

[0010] The present disclosure has been made in view of such a situation, and an object of the present disclosure is to provide a steel sheet for hot stamping capable of obtaining a hot stamp member having an excellent collision energy absorbing effect, and a hot stamp member having an excellent collision energy absorbing effect.SOLUTIONS TO THE PROBLEMS

[0011] A first aspect of the present invention is a steel sheet for hot stamping, containing: C: 0.050 to 0.12 mass%; Si: 0.50 to 2.0 mass%; Mn: 2.1 to 3.0 mass%; P: 0.10 mass% or less (including 0 mass%); S: 0.010 mass% or less (including 0 mass%); Al: 0.01 to 0.10 mass%; Ti: 0.010 to 0.100 mass%; B: 0.0010 to 0.0100 mass%; N: 0.010 mass% or less (including 0 mass%); and a residual portion being Fe and inevitable impurities, and satisfying the following formula (1). α = % C / 21 − % Si / 334 + % Mn / 81 − % Ti / 11 + 10 × % B ≥ 0.03 Herein, [%C], [%Si], [%Mn], and [%Ti] are the contents of C, Si, Mn, and Ti represented in mass%, respectively, and <%B> is the amount of solid solution B represented in mass%.

[0012] A second aspect of the present invention is the steel sheet for hot stamping according to the first aspect, in which the steel sheet for hot stamping is a hot-rolled steel sheet.

[0013] A third aspect of the present invention is the steel sheet for hot stamping according to the first aspect, in which the steel sheet for hot stamping is a cold-rolled steel sheet.

[0014] The fourth aspect of the present invention is a hot stamp member with the steel sheet for hot stamping according to any one of the first to third aspects, in which a metal microstructure has a total area ratio of martensite and bainite of 70% or more and an area ratio of ferrite of 30% or less, and a tensile strength is 800 MPa or more and 1300 MPa or less. EFFECTS OF THE INVENTION

[0015] One embodiment of the present invention can provide a steel sheet for hot stamping capable of providing a hot stamp member having an excellent collision energy absorbing effect, and another embodiment of the present invention can provide a hot stamp member having an excellent collision energy absorbing effect.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] [Fig. 1] Fig. 1 shows a view of a sectional shape of a hot stamp member according to an embodiment. [Fig. 2] Fig. 2 shows a schematic view of a crushing test body. DETAILED DESCRIPTION

[0017] The present inventors have conducted intensive studies in order to solve the above problems. In particular, as a result of detailed studies focusing on the microstructure after hot stamping, it has been found that it is necessary to suppress the amount of ferrite in the metal microstructure in the state after hot stamping, that is, in the state of the hot stamp member. More specifically, it has been found that not only the content of each of C, Si, Mn, P, S, Al, Ti, B, and N is set within a predetermined range, but also the following formula (1) is satisfied, thereby allowing the metal microstructure of the hot stamp 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 absorbing effect can be obtained. α = % C / 21 − % Si / 334 + % Mn / 81 − % Ti / 11 + 10 × % B ≥ 0.03

[0018] Herein, [%C], [%Si], [%Mn], and [%Ti] are the contents of C, Si, Mn, and Ti represented in mass%, respectively, and <%B> is the amount of solid solution B represented in mass%.

[0019] When using the steel sheet according to the embodiment of the present invention in which the content of each of C, Si, Mn, P, S, Al, Ti, B, and N is set to fall within a predetermined range and the value of α determined by the formula (1) is set to fall within a predetermined range, a hot stamping material obtained when not special hot stamping conditions but general hot stamping conditions are applied can have an excellent collision energy absorbing effect.

[0020] Hereinafter, each requirement defined in the embodiments of the present invention will be described in detail.1. Chemical composition

[0021] The steel sheet and the hot stamp member according to the embodiment of the present invention can have an excellent collision energy absorbing effect after hot stamping (that is, the state of the hot stamp member) by having the chemical composition described below.[C: 0.050 mass% or more and 0.12 mass% or less]

[0022] In the present invention, C is an important element. In order to suppress excessive increase in strength, securing a certain degree of hardenability, and suppress the generation of ferrite, it is necessary to set the amount of C to an appropriate range. Excessively reducing the amount of C fails to secure the strength and hardenability of the hot stamp member, and thus the amount of C is set to 0.050 mass% or more. The amount of C is preferably 0.060 mass% or more, and more preferably 0.070 mass% or more. In contrast, the excessive amount of C leads to the excessive strength after hot stamping, and the toughness of the hot stamp member is insufficient, thereby causing cracks due to deformation at the time of collision to easily occur and develop and the collision energy absorption properties to be deteriorated, and thus the upper limit of the amount of C is set to 0.12 mass%. The amount of C is preferably 0.11 mass% or less, and more preferably 0.10 mass% or less.[Si: 0.50 mass% or more and 2.0 mass% or less]

[0023] Si is an important element for securing the strength of the hot stamp member. Adding Si suppresses precipitation of C as a carbide during cooling after hot stamping, thereby allowing strength to be secured. In order to exhibit such an effect, the amount of Si is set to 0.50 mass% or more. The amount of Si is preferably 0.60 mass% or more, and more preferably 0.70 mass% or more. However, an excessive amount of Si causes deterioration of pickling properties due to an increase in oxide scale during production of a steel sheet, an increase in Ac 3 transformation temperature, a decrease in cross tensile strength of a spot welded portion, and the like. Therefore, the amount of Si is 2.0 mass% or less. The amount of Si is preferably 1.8 mass% or less, and more preferably 1.6 mass% or less.[Mn: 2.1 mass% or more and 3.0 mass% or less]

[0024] In the present invention, Mn is an important element. It is useful for improving the hardenability of the steel sheet and making the main phase of the metal microstructure after hot stamping martensite and bainite while the amount of C is adjusted to the above range. In order to exhibit such an effect, the amount of Mn is 2.1 mass% or more. However, when the amount of Mn exceeds 3.0 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 the cold rollability thereafter at the time of producing the steel sheet before hot stamping. Therefore, the amount of Mn is 3.0 mass% or less. The amount of Mn is preferably 2.8 mass% or less, and more preferably 2.6 mass% or less.[P: 0.10 mass% or less (including 0 mass%)]

[0025] P is an inevitable impurity element and is an element that deteriorates weldability of the steel sheet, and thus it is desirable to limit the content thereof as much as possible. In order not to deteriorate weldability of the steel sheet, the amount of P is set to 0.10 mass% or less. The amount of P is preferably 0.050 mass% or less, and more preferably 0.020 mass% or less. P is an impurity inevitably mixed in steel, and it is difficult to set the amount thereof to 0 mass% in industrial production, and an amount of 0.0005 mass% or more is typically contained.

[0026] In the present specification, "including 0 mass%" means including an embodiment in which the elements are not intentionally added, that is, a case of the content being equal to or less than the inevitable impurity level (the case in which the elements are intentionally added is not excluded).[S: 0.010 mass% or less (including 0 mass%)]

[0027] S is an inevitable impurity element, and deteriorates weldability of the steel sheet. Therefore, the amount of S is 0.010 mass% or less. The amount of S is preferably 0.0080 mass% or less, and more preferably 0.0050 mass% or less. The S amount is preferably as small as possible, but it is difficult to set the S amount to 0 mass% in industrial production, and an amount of 0.0001 mass% or more is typically contained.[Al: 0.01 mass% or more and 0.10 mass% or less]

[0028] Al is an element that acts as a deoxidizer. In order to exhibit such an effect, the amount of Al is set to 0.01 mass% or more. The amount of Al is preferably 0.015 mass% or more, and more preferably 0.020 mass% or more. However, excessive Al content leads to an increase in production cost, and thus the amount of Al is set to 0.10 mass% or less. The amount of Al is preferably 0.08 mass% or less, and more preferably 0.07 mass% or less.[Ti: 0.010 mass% or more and 0.100 mass% or less]

[0029] Ti is an important element in the present invention. Containing Ti promotes the formation of TiN, thereby increasing the amount of solid solution B that improves hardenability (it is possible to suppress bonding of N to B). In order to exhibit such an effect, the amount of Ti is 0.010 mass% or more. The amount of Ti is preferably 0.012 mass% or more, and more preferably 0.015 mass% or more. However, the excessive amount of Ti causes TiC to be easily generated, the steel structure to be refined, and hardenability to be rather deteriorated. Therefore, the amount of Ti is 0.100 mass% or less, preferably 0.095 mass% or less, and more preferably 0.090 mass% or less.[B: 0.0010 mass% or more and 0.0100 mass% or less]

[0030] In the present invention, B is an important element. B is known as an element that strengthens a grain boundary, and is contained as an element effective for improving hardenability. In order to obtain such an effect, the amount of B needs to be 0.0010 mass% or more. The amount of B is preferably 0.0012 mass% or more, and more preferably 0.0015 mass% or more. However, the excessive amount of B becomes a factor of causing surface cracking or the like at the time of casting due to generation of the B compound or the like. Therefore, the amount of B needs to be 0.0100 mass% or less, preferably 0.0090 mass% or less, and more preferably 0.0085 mass% or less.[N: 0.010 mass% or less (including 0 mass%)]

[0031] N is an inevitable impurity element, and when N is excessively contained, BN is easily generated, and the amount of solid solution B is reduced. Therefore, the amount of N is 0.010 mass% or less. The amount of N is preferably 0.008 mass% or less, and more preferably 0.005 mass% or less. The amount of N is preferably as small as possible, and thus the lower limit is not particularly limited, but it is difficult to set the amount to 0 mass% in industrial production, and an amount of 0.0001 mass% or more is typically contained.[Residual portion]

[0032] The basic components of the steel sheet for hot stamping and the hot stamp member according to the embodiment of the present invention are as described above, and the residual portion is iron and inevitable impurities. Examples of the inevitable impurities include, in addition to O that is preferably suppressed as described below, and tramp elements (Pb, Bi, Sb, Sn, V, and the like) brought depending on situations of raw materials, materials, production facilities, and the like within a range in which the effects of the present invention are not impaired.

[0033] There are elements such as P, S, and N, which are typically preferable as the content is smaller, and thus are inevitable impurities, but are separately defined as the composition range as described above. Therefore, in the present specification, the "inevitable impurities" constituting the residual portion indicate a concept excluding elements with composition range separately defined.

[0034] O is an inevitable impurity element, and when excessively included, an oxide is formed and the solid solution Si is reduced. Therefore, the amount of O is preferably 0.010 mass% or less. The amount of O is more preferably 0.005 mass% or less, and still more preferably 0.003 mass% or less. The amount of O is preferably as small as possible, and thus the lower limit is not particularly limited, but it is difficult to set the amount to 0 mass% in industrial production, and an amount of 0.0001 mass% or more is typically contained.[Value α of formula (1)]

[0035] In order to set the metal microstructure of the hot stamp member obtained when general hot stamping conditions are applied to the steel sheet for hot stamping of the present invention to 70% or more in total of martensite and bainite and 30% or less of ferrite in terms of area ratio, it is necessary not only to set the content of each of C, Si, Mn, P, S, Al, Ti, B, and N within the above predetermined range but also to satisfy the following formula (1). α = % C / 21 − % Si / 334 + % Mn / 81 − % Ti / 11 + 10 × % B ≥ 0.03

[0036] Herein, [%C], [%Si], [%Mn], and [%Ti] are the contents of C, Si, Mn, and Ti represented in mass%, respectively, and <%B> is the amount of solid solution B represented in mass%.

[0037] The value α obtained by the above formula (1) is an index indicating hardenability of the steel sheet. Ferrite fractions of various samples obtained by applying general hot stamping conditions were measured, 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.

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

[0039] In view of the fact that the denominator of the coefficient of the term Nb is small, and it is difficult to secure α ≥ 0.03 in a useful amount of Nb added, Nb is not contained in the formula (2) (that is, no Nb addition), thereby achieving the formula (1).

[0040] The amount of the solid solution B, <%B>, can be determined as follows.

[0041] N present in the steel more readily bonds with Ti than B. Under the condition that all N is bonded to Ti to form TiN, that is, when the following formula (3) is satisfied because of the atomic weights 47.8 and 14 of Ti and N, respectively and since TiN is 1:1 in the stoichiometric ratio, all N are converted to TiN, BN is not formed, and all added B (B amount) is determined to be a solid solution B. That is, the amount of the solid solution B, <%B>, is equal to the B amount. % Ti ≥ 47.8 / 14 × % N

[0042] Herein, [%Ti] and [%N] are the contents of Ti and N represented in mass%, respectively.

[0043] In contrast, when the above formula (3) is not satisfied, N that has not formed TiN forms BN, and thus the amount of the 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 the formula (4) is negative, <%B> is set to zero. % B = 10.8 × % B / 10.8 − % N / 14 − % Ti / 47.8

[0044] Herein, [%B], [%Ti], and [%N] are the contents of B, Ti, and N represented in mass%, respectively.

[0045] The "general hot stamping conditions" described above are conditions that satisfy (1) heating to a soaking temperature of the Ac 3 temperature or more and holding at the soaking temperature (soaking), (2) an average cooling rate from the soaking temperature to the molding temperature, the cooling rate being equal to or more than a cooling rate by natural cooling, and (3) a molding temperature of 500 to 750°C. The "molding temperature" means the temperature of the steel sheet immediately before molding with a mold, and the "equal to or more than a cooling rate by natural cooling" means that than a cooling rate when the steel sheet is taken out of the heating furnace and then is naturally cooled in the air without heating for lowering the cooling rate.2. Metal microstructure(1) Metal microstructure of steel sheet for hot stamping

[0046] The steel sheet for hot stamping according to the embodiment of the present invention is a steel sheet that can be processed into a desired shape by applying the above-described "general hot stamping conditions". Heating is performed to a temperature of the Ac 3 temperature or more during hot stamping, and thus there is no particular restriction on the metal microstructure in the state of the steel sheet. The steel sheet for hot stamping according to the embodiment of the present invention may be a hot-rolled steel sheet or a cold-rolled steel sheet.

[0047] The hot-rolled steel sheet does not need to be subjected to cold rolling after hot rolling, and is excellent in productivity. In contrast, the cold-rolled steel sheet has an advantage that a thin steel sheet can be easily obtained.(2) Hot stamp member

[0048] The hot stamp member according to the embodiment of the present invention can be obtained by applying "general hot stamping conditions" to the above-described steel sheet for hot stamping according to the embodiment of the present invention.

[0049] The metal microstructure of the obtained hot stamp member has 70% or more in total of martensite and bainite, 30% or less of ferrite, and less than 1% of other microstructures in an area ratio. Examples of other microstructures include pearlite and retained austenite. Preferably, other microstructures are not included. That is, preferably, the metal microstructure includes only 70% or more in total of martensite and bainite and 30% or less of ferrite in an area ratio.

[0050] 70% or more in total of martensite and bainite in an area ratio allows the tensile strength of the obtained hot stamp member to be 800 MPa or more and 1300 MPa or less. This makes it possible to secure a sufficient energy absorption amount at the time of collision. The total of martensite and bainite is preferably 80% or more, more preferably 90% or more in an area ratio.

[0051] 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 asquenched martensite without carbide, tempered martensite with carbide, and selftempered martensite. The bainite may be either or both of upper bainite including cementite and retained austenite between laths and lower bainite including carbides within laths.

[0052] Ferrite is an equiaxed crystal grain including no iron-based carbide. Setting the area ratio of ferrite to 30% or less makes it possible to reduce a structure in which strain due to deformation at the time of collision is concentrated and to suppress the occurrence and development of cracks at an early stage. The area ratio of ferrite is preferably 20% or less, and more preferably 10% or less.

[0053] The metal microstructure is observed at a plate thickness t / 4 position (a position at a distance of 1 / 4 of the sheet thickness t from the surface (main surface) of the steel sheet constituting the hot stamp member toward the center), which is a representative portion, from the flat portion (portion having no hot deformation such as bending) of the hot stamp member.

[0054] The surfaces of the steel sheet for hot stamping and the hot stamp member according to the embodiment of the present invention may or may not have a plated layer such as hot-dip galvanized coating or alloyed zinc coating. Having such a plated layer, for example, allows scale generation at the time of hot stamping heating to be suppressed. In contrast, when the plated layer is not provided, the production cost of the steel sheet can be suppressed. In addition, the steel sheet for hot stamping and the hot stamp member according to the embodiment of the present invention may have a decarburized layer on the surface in order to improve moldability at the time of hot stamping.3. Production method

[0055] In the steel sheet for hot stamping according to the embodiment of the present invention, the metal microstructure is not particularly limited as described above. Therefore, as long as the chemical composition defined in the above "1. Chemical composition" including the formula (1) is satisfied, the production of the hot-rolled steel sheet and the cold-rolled steel sheet can be easily performed by using a known method.

[0056] The steel sheet for hot stamping according to the embodiment of the present invention can be obtained by performing hot stamping under conditions that satisfy the above-described "general hot stamping conditions": (1) heating to a soaking temperature of the Ac 3 temperature or more and holding at the soaking temperature (soaking); (2) an average cooling rate from the soaking temperature to a forming temperature, being more than a cooling rate by natural cooling; and (3) a forming temperature of 500 to 750°C.

[0057] Examples of the soaking temperature of the Ac 3 temperature or more include a temperature of 850 to 1000°C. In addition, examples of the soaking time (holding time) for holding at such a soaking temperature include 50 to 300 seconds.

[0058] Further, examples of "more than a cooling rate by natural cooling" which is an average cooling rate from the soaking temperature to the molding temperature include natural cooling (for example, 2 °C / s) to 50°C / s.EXAMPLE1. Example 1

[0059] In Example 1, the relationship between the value of α obtained by the composition and the formula (1), and the metal microstructure of the sample after hot stamping (hot stamp member) was clarified.(1) Preparation of sample of steel sheet for hot stamping

[0060] Steel having the components shown in Table 1 was melted to form a slab. In addition to the chemical composition, Table 1 also shows the value α calculated using the formula (1). For the steel types A, D, E, and I containing B, the value of the right side of the formula (3) and whether or not the formula (3) is satisfied were described. As is found from Table 1, all the steel types A, D, E, and I satisfy the formula (3), and thus the amount B of each steel type was used as the value of the B amount of solid solution, , in the α calculation using the formula (1).

[0061] The obtained slab was heated to 1100 to 1300°C, hot-rolled such that the finish rolling temperature was 890 to 950°C, and held at 500 to 700°C. After the holding, cooling to room temperature was performed to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was pickled to remove surface scale, and then cold-rolled to produce a cold-rolled steel sheet. The sheet thickness of the cold-rolled steel sheet sample 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 necessary, and hot-dip galvanizing treatment or alloying hot-dip galvanization was performed. [Table 1]Chemical composition (mass%), residual portion: Fe and inevitable impuritiesαValue of right side in formula (3)Satisfaction / dissatisfaction of formula (3)Steel typeCSiMnPSAlTiBNbNA0.0851.182.230.0110.0020.0360.0250.001600.00310.0420.0106SatisfactionB0.0531.24*0.210.0100.0020.035*0*000.0031*0.0014--C*0.0461.21*0.210.0100.001*0.487*0*000.0040*0.0012--D0.077*0.35*1.590.0100.0020.0360.0220.0022*0.0500.0027*0.0170.0092SatisfactionE0.060*0.02*1.540.0090.0020.0350.0710.0015*0.0510.0027*0.00490.0092SatisfactionF0.069*02.100.0100.0030.045*0*0*0.0500.0020*0.0042--G0.080*0.03*1.450.0170.0010.035*0*0*0.0320.0020*0.0056--H0.0821.132.270.0100.0010.0440.0290.001900.00490.0450.0167SatisfactionI*0.2201.152.230.0100.0010.0400.0310.002000.00250.0520.0085Satisfaction* means outside the scope defined by the embodiments of the present invention.

[0062] For the steel plate sample thus obtained, a blank having a size shown in Table 2 was cut out, and hot stamping was performed on the obtained blank under the conditions shown in Table 2 (soaking temperature, soaking time, average cooling rate from soaking temperature to molding temperature, and molding temperature) to obtain a hot stamp member sample. [Table 2]Steel typeSample nameSheet thickness [mm]Blank size [mm]Soaking temperature [°C]Soaking time [s]Average cooling rate from soaking temperature to molding temperature [°C / s]Molding temperature [°C]Ferrite fraction after hot stamping [area%]AA11.4150×250890200305500A21.4180×709001009.57000BB11.4150×25089020030550*97B21.4180×709001009.5700*100CC11.4150×25089020030550*100C21.4180×709001009.5700*96DD11.4150×25089020030550*56D21.4180×709001009.57009EE11.4150×25089020030550*88E21.4180×709001009.5700*71FF11.2150×25089030020750*90F21.2150×250890300Natural cooling750*83GG11.4150×25089030020750*88G21.4150×250890300Natural cooling750*86HH11.2217×30093018012.35500H21.4150×250890200305500II11.2217×30093018012.35500121.4150×250890200305500* means that the ferrite fraction exceeds 30% in an area ratio.

[0063] The results of the microstructure observation and the measurement of the ferrite fraction of the obtained hot stamp member sample are shown in Table 2.

[0064] The microstructure observation was performed by observing a region of 100 µm in length × 130 µm in width in a visual field at a sheet thickness t / 4 position on a surface parallel to the rolling direction, which had been subjected to nital etching, with a scanning electron microscope (SEM) at a magnification of 1000 to 3000.

[0065] No microstructures other than ferrite, martensite, and bainite were observed in any of the samples. That is, the microstructures other than ferrite were all martensite or bainite.

[0066] The ferrite fraction was determined by a point counting method in which grids at 100 or more intersections of vertical lines and horizontal lines were uniformly arranged a 1000 to 3000 times SEM image subjected to microstructure observation, and the number of intersections on each phase was counted.

[0067] 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 value of α.

[0068] In the 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 more than 30%.

[0069] In the sample D-2, Si and Mn are lower than specified values. Also due to this influence, the ferrite fraction was 30% or less although the value of α deviated. However, the value of Si is low, and thus it is considered that sufficient strength cannot be obtained.

[0070] Samples I1 and I2 had a value of α within the defined range and a ferrite fraction of 0. However, the amount of C is excessively large, the strength is excessively high, and sufficient collision energy absorption properties cannot be obtained (refer to also Example 2).2. Example 2

[0071] In Example 2, in addition to Samples H1 and I1 of Example 1, three types of general-purpose high-strength steel sheet (cold-rolled steel sheets) for cold forming (steel types J, K, and L) with the chemical compositions thereof shown in Table 3 as comparative materials were evaluated for collision energy absorption properties. [Table 3]Chemical composition (mass%), residual portion: Fe and inevitable impuritiesSteel typeCSiMnPSAlTiBNbJ0.070*02.050.0180.0030.045*0*00High-strength steel sheet for cold formingK0.123*0.102.670.0100.0020.0450.070*00High-strength steel sheet for cold formingL*0.1701.352.100.0150.0010.0400.030*00High-strength steel sheet for cold forming* means outside the scope defined by the embodiments of the present invention. (1) Preparation of sample for crushing test

[0072] Blank samples for samples H1-1 and H1-2 having a width of 217 mm × a length of 300 mm × 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 blank for Sample H1-1 and the blank for Sample H1-2 are the same except that the former is not subjected to plating and the latter is subjected to alloyed zinc coating.

[0073] Blank samples for samples I1-1 and I1-2 having a width of 217 mm × a length of 300 mm × a thickness of 1.2 mm were prepared as in Example 1 in the same manner as in I1 of Example 1. As shown in Table 4, the blank for Sample I1-1 and the blank for Sample I1-2 are the same except that the former is subjected to alloyed zinc coating, and the latter has a surface decarburized layer in addition to the alloyed zinc coating.

[0074] Using these blank samples, a hot stamp member sample having the sectional shape of Fig. 1 was prepared as in Example 1. The hot stamping conditions for Samples H1-1 and H1-2 are the same as Sample H1 in Example 1, and the hot stamping conditions for Samples I1-1 and I1-2 are the same as Sample I1 in Example 1.

[0075] In addition, samples for cold forming for samples J-1, K-1, and L-1 having a width of 217 mm × a length of 300 mm × a thickness of 1.2 mm were cut out from the high-strength steel sheet samples for cold forming of the steel types J, K, and L, respectively. The obtained cold-forming sample was cold- to obtain a cold-formed sample of the high-strength steel sheet having the cross-section of Fig. 1.

[0076] Then, both ends of the obtained hot stamp member and the cold-formed sample of the high-strength steel sheet (the hot stamp member and the cold-formed sample of the high-strength steel sheet may be collectively referred to as a "hat member") were cut to have a length of 200 mm. Further, a steel sheet of 590 Ma class having 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.

[0077] Further, a top sheet was attached to an upper end portion of the hat member by arc welding, and a bottom sheet was attached to a lower end portion of the hat member by arc welding. The top sheet had a size of 200 mm × 200 mm, the bottom sheet had a size of 250 mm × 250 mm, and the sheet thickness was 9 mm in both cases. Both the top sheet and the bottom sheet were positioned such that the center of gravity coincided with the hat member.

[0078] Fig. 2 shows a schematic view of a crushing test body obtained in this manner. In Fig. 2, the sheet attached to the flange portion of the hat member by spot welding is omitted such that the shape of the hat member can be easily recognized, but the positions of spot welding provided at intervals of 25 mm are shown.(2) Crushing test

[0079] Using the obtained crushing test body, a dynamic axial crushing test was performed in which a falling weight having a weight of 190 kg was dropped from a height position of 10 m. A stopper for receiving a falling weight was placed at a position of 105 mm downward from the top sheet of the crushing test body. The load was measured using a load cell, and the displacement was marked on the falling weight and measured with a laser displacement meter.

[0080] A point where the falling weight was in contact with the crushing test body and the load was recorded in the load cell was defined as a stroke of 0%, and a point where the displacement was 100 mm was defined as a stroke of 100%. The recorded data were averaged at a moving average of 21 points.

[0081] The load at strokes of 0 to 100% and the load at strokes of 80 to 100% were integrated to calculate the absorbed energy amount of each load.

[0082] The value obtained by dividing the absorbed energy amount at strokes of 80 to 100% by the absorbed energy amount at strokes of 0 to 100% was represented in % to provide the EA R . The test was performed at least three times on each sample, and the average value thereof was shown in Table 4 as the measurement result of each sample. Samples having a total absorbed energy amount of 5.5 kJ or more and an EA R of 18% or more were determined to have high absorbed energy.

[0083] 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 in a bellows shape that was a normal crushing form was described as "bellows deformation" in Table 4.

[0084] Samples having a total absorbed energy amount of 5.5 kJ or more, an EA R of 18% or more, and a crushing form of all the samples being bellows deformation were determined to have excellent collision energy absorption properties. [Table 4]Steel typeSample nameSample form High-strength steel sheet for cold formingFerrite fraction [%]Surface propertySheet thickness [mm]Tensile strength [MPa]Absorbed energy amount at strokes of 80 to 100% [kJ]Total absorbed energy amount [kJ]EA R [%]Crushing formHH1-1Hot stamping0Non-plating1.211681.467.9318.4Bellows deformationH1-2Hot stamping0Alloyed galvanization1.211241.357.0819.0Bellows deformationII1-1Hot stamping0Alloyed galvanization1.2*16230.956.4814.6Large crack occurred and led to fractureI1-2Hot stamping0Alloyed galvanization, surface decarburization1.2*15531.107.2415.2Large crack occurred and led to fractureJJ-1* High-strength steel sheet for cold forming*86Alloyed galvanization1.2*6170.954.7819.9Bellows deformationKK-1* High-strength steel sheet for cold forming*62Alloyed galvanization1.210360.906.3614.2Large crack occurred and led to fractureLL-1* High-strength steel sheet for cold forming*32Non-plating1.212511.367.8517.3Large crack occurred and led to fracture* means outside the scope defined by the embodiments of the present invention. (Strength)

[0085] A No. 5 test piece (JIS No. 5 test piece) defined by JIS Z 2241 was taken from a flat portion (portion having no hot deformation such as bending) of the hat member of each sample, and the tensile strength was measured. The measurement results are shown in Table 4.

[0086] Samples H1-1 and H1-2 according to embodiments of the present invention showed excellent collision absorption energy properties.

[0087] The members of Samples I1-1 and I1-2 had excessively high tensile strength, had a low EA R in the crushing test, and fractured due to generation of large cracks.

[0088] Sample J-1 had low tensile strength and low total absorbed energy amount.

[0089] Samples K-1 and L-1 had a low EA R in the crushing test, and fractured due to generation of large cracks.

[0090] This application claims priority based on a Japanese patent applicaiton, JP-2023-062285 filed on April 6, 2023. JP-2023-062285 is incorporated herein by reference.

Claims

1. A steel sheet for hot stamping, comprising: C: 0.050 to 0.12 mass%; Si: 0.50 to 2.0 mass%; Mn: 2.1 to 3.0 mass%; P: 0.10 mass% or less (including 0 mass%); S: 0.010 mass% or less (including 0 mass%); Al: 0.01 to 0.10 mass%; Ti: 0.010 to 0.100 mass%; B: 0.0010 to 0.0100 mass%; N: 0.010 mass% or less (including 0 mass%); and residual portion being Fe and inevitable impurities, and satisfying a following formula (1): α = % C / 21 − % Si / 334 + % Mn / 81 − % Ti / 11 + 10 × % B ≥ 0.03 wherein [%C], [%Si], [%Mn], and [%Ti] are contents of C, Si, Mn, and Ti represented in mass%, respectively, and <%B> is an amount of solid solution B represented in mass%.

2. The steel sheet for hot stamping according to claim 1, wherein the steel sheet for hot stamping is a hot-rolled steel sheet.

3. The steel sheet for hot stamping according to claim 1, wherein the steel sheet for hot stamping is a cold-rolled steel sheet.

4. A hot stamp member with the steel sheet for hot stamping according to any one of claims 1 to 3, wherein a metal microstructure has a total area ratio of martensite and bainite of 70% or more and an area ratio of ferrite of 30% or less, and a tensile strength is 800 MPa or more and 1300 MPa or less.

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