pame of Document] DESCRlPTION [Title of the Invention] STEEL SHEET FOR HOT STAMPING, METHOD FOR PRODUCTION THEREOF, AND HOT STAMPING STEEL MATERIAL [Technical Field] 5 [OOOl] The present invention relates to a steel sheet for hot stamping, a method for production thereof, and a hot stamping steel material. [Background Art] [0002] 10 In the field of transportation equipment such as automobiles, an attempt is extensively made to reduce the mass by using high-strength materials. For example, in automobiles, use of high-strength steel sheets has been steadily increased with an intention to improve collision safety and enhance functionality without increasing the car body mass, and also improve fuel efficiency to reduce emissions of carbon dioxide. 15 [0003] In this movement for expansion of use of high-strength steel sheets, the biggest problem is manifestation of a phenomenon called "degradation of shape fixability", which is more likely to occur as the strength of the steel sheet is increased. The phenomenon is more likely to occur as the spring back amount after forming increases with strength 20 enhancement, and the. phenomenon causes such an additional problem specific to high-strength steel sheets that it is not easy to obtain a desired shape. [0004] For solving the problem, it is necessaly in a usual method for forming a high-strength steel sheet additionally to carry out an unnecessary processing step (e.g. 25 restriking) for a low-strength material free from the problem of degradation of shape fixability, or to chanie the product shape. As one method for solving such situations, a hot-forming method called a hot stamping method has received attention. The hot stamping method is a method in which a steel sheet (processed material) is heated to a predetermined temperature (generally the 5 temperature that serves as an austenite phase), and stamped by a die having a temperature (e.g. room temperature) lower than the temperature of the processed material with the strength of the processed material decreased for facilitating forming, whereby a desired shape can be easily provided, and also a rapid cooling heat treatment (quenching) using a difference in temperature between the processed material and the pressing is performed to 10 increase the strength of a product after forming. [0006] In recent years, the hot stamping method has been recognized for its usefulness, and a wide range of steel materials have been considered to be applied. Examples thereof include steel materials that are used under a severe corrosive environment, like automobile 15 undercarriage components, and steel materials provided with perforated portions for the purpose of joining other components. Thus, steel materials obtained by tlie hot stamping method have been required to have not only strength but also hydrogen embrittlement resistance. [0007] 20 This is because while it is generally known that hydrogen embrittlement resistance is reduced with strength enhancement of steel materials, a steel material obtained by the hot stamping method generally has high strength, and therefore in application of the hot stamping method to the steel material, the steel material is exposed to a corsosive environment to accelerate ingress of hydrogen into the steel, and massive 25 residual stress occurs as processing such as punching is performed, thus raising the possibility that hydrogen ernbrittlement occurs. From such a viewpoint, a technique intended to secure hydrogen embrittlement resistance has also been proposed for steel materials whose strength is enhanced by the hot stamping method. For example, Patent Literature 1 discloses a technique concerning a 5 steel sheet having resistance to delayed rupture (the same meaning as hydrogen embrittlement resistance) by including at a predetermined density one or more of oxides, sulfides, composite crystallized products and composite precipitated products of Mg having an average particle size in a predetermine range. Patent Literature 2 discloses a technique in which the punching characteristic is improved by performing punching 10 (perforation) in a high-temperature state (hot) after heating for hot stamping and before pressing, so that delayed rupture resistance is improved. [Prior Art Literatures] [Patent Literatures] [0009] 15 [Patent Literature 11 JP2006-9116A [Patent Literature 21 JP20 10- 174291 A [Patent Literature 31 JP2006-29977A [Summary of the Invention] [Problems to Be Solved by the Invention] 20 [OOlO] Although the technique disclosed in Patent Literature 1 is an excellent technique, but it is a technique in which Mg that is not easily included in general is made to exist in the steel, and a product containing Mg is highly controlled. Therefore, a more easily practicable technique is desired. 25 [OOll] The technique disclosed in Patent Literature 2 is a technique based on hot perforation in which punching (perforation) is performed in a high-temperature state (hot) after heating for hot stamping and before pressing. Accordingly, high dimensional accuracy cannot be secured in a steel material after hot stamping. Further, the shape capable of being formed by the technique is restricted. Therefore, it is difficult to expand 5 the range of applications (components) of the hot stamping method by the technique disclosed in Patent Literature 2. [0012] Thus, there has not been proposed a technique which secures good hydrogen embrittlement resistance even when processing leading to remaining of stress, such as 10 perforation, is performed after hot stamping and which is easily practicable. [0013] Accordingly, an object of the present invention is to provide a steel sheet for hot stamping, which secures good hydrogen e~nbrittlement resistance even when a steel material after hot stamping is subjected to processing leading to remaining of stress, such 15 as perforation; a method for production thereof which can easily be performed; and a hot stamping steel material. [Means for Solving the Problems] [0014] For achieving the object described above, the present inventors have extensively 20 conducted studies as described below. The present inventors have given attention to a Mn-containing inclusion and a Mn oxide which are relatively easily generated in the steel, and come up with a new idea of securing good hydrogen embrittlement resistance by making these substances serve as a trap site for diffusible hydrogen and non-diffusible hydrogen. 25 [0015] Then, steel sheets for hot stamping have been prepared under various conditions and subjected to a hot stamping method, and for the obtained steel materials, strength and ductility as fundamental characteristics as well as hydrogen embrittlement resistance and toughness have been examined. As a result, it has been newly found that good hydrogen embrittlement resistance can be secured in the steel material after hot stamping by 5 increasing the concentration of the Mn-containing inclusion and the number ratio of the Mn oxide to the Mn-containing inclusion having a predetermined size. [0016] On the other hand, such a problem has been newly found that when the concentration of the Mn-containing inclusion is excessively increased, a reduction in 10 toughness becomes apparent in the steel material after hot stamping. That is, it has been newly found when the concentration of the Mn-containing inclusion falls within a predetermined range and the number density of the Mn oxide to the Mn-containing inclusion having a predetermined size is equal to or greater than a predetermined value, good hydrogen embrittlement resistance can be secured and good toughness can be secured 15 even when the steel material after hot stamping is subjected to processing leading to remaining of stress, such as punching. [0017] Then, it has been newly found that by increasing the coiling temperature in a hot rolling step as compared to conventional techniques and performing cold rolling in 20 conditions for production of the steel sheet for hot stamping, the concentration of the Mn-containing inclusion can be made fall within a predetermined range and the number ratio of the Mn oxide to the Mn-containing inclusion having a predetermined size can be made equal to or greater than a predetermined value. [OOIS] 25 The present invention has been devised based on the above-described new findings, and the subject thereof is as follows. (1) A steel sheet for hot stamping, wherein the steel sheet has the chemical composition of: C: 0.18 to 0.26%; Si: more than 0.02% and not more than 0.05% ; Mn: 1.0 to 1.5%; P: 0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less; 0: 0.0010 to 0.020%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; 5 Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and impurities, in terms of % by mass, the concentration of a Mn-containing inclusion is not less than 0.010% by mass and less than 0.25% by mass, and the number ratio of a Mn oxide to the inclusion having a maximum length of 1.0 to 4.0 pm is 10.0% or more. too1 91 10 (2) The steel sheet for hot stamping according to (I), wherein the chemical composition includes one or more selected from the group consisting of Cr: 0.01 to 2.0%; Mo: 0.01 to 1.0%; V: 0.01 to 0.5%; W: 0.01 to 0.5%; Ni: 0.01 to 5.0%; and B: 0.0005 to 0.01%, in terms of % by mass. [0020] 15 (3) The steel sheet for hot stamping according to (1) or (2), wherein the chemical composition includes one or more selected from the group consisting of Ti: 0.001 to 0.5%; Nb: 0.001 to 0.5%; and Cu: 0.01 to 1.0%, interms of % by mass. [0021] (4) The steel sheet for hot stamping according to any one of (1) to (3), wherein the 20 steel sheet includes on a surface thereof an aluminum hot-dipping layer having a thickness of 50 pm or less. [0022] (5) The steel sheet for hot stamping according to any one of (1) to (3), wherein the steel sheet includes on a surface thereof a hot-dip galvanized layer having a thickness of 30 25 pm or less. [0023] (6) The steel sheet for hot stamping according to any one of (1) to (3), wherein the steel sheet includes on a surface thereof an alloyed hot-dip galvanized layer having a thickness of 45 pm or less. [0024] 5 (7) A method for production of a steel sheet for hot stamping, the method including: a hot rolling step of hot-rolling a steel piece having the chemical composition of: C: 0.18 to 0.26%; Si: more than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P: 0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less; 0: 0.0010 to 0.020%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; 10 Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and impurities, in terms of % by mass, and then coiling the steel piece at a temperature of 690°C or higher to form a hot-rolled steel sheet; and a cold rolling step of cold-rolling the hot-rolled steel sheet at a draft of 10 to 90% to form a cold-rolled steel sheet. [0025] 15 (8) The method for production of a steel sheet for hot stamping according to (7), wherein the chemical composition includes one or more selected from the group consisting of Cr: 0.01 to 2.0%; Mo: 0.01 to 1.0%; V: 0.01 to 0.5%; W: 0.01 to 0.5%; Ni: 0.01 to 5.0%; and B: 0.0005 to 0.01%, in terms of % by mass. [0026] 20 (9) The method for production of a steel sheet for hot stamping according to (7) or (a), wherein the chemical composition includes one or more selected from the group consisting of Ti: 0.001 to 0.5%; Nb: 0.001 to 0.5%; and Cu: 0.01 to 1.0%, in terms of % by mass. [0027] 25 (10) A method for production of a steel sheet for hot stamping, wherein the steel sheet for hot stamping, which is obtained by the production method according to any one of (7) to (9), is immersed in an aluminum hot-dipping bath to form an aluminum hot-dipping layer on the surface of the steel sheet. [0028] (11) A method for production of a steel sheet for hot stamping, wherein the steel 5 sheet for hot stamping, which is obtained by the production method according to any one of (7) to (9), is immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the steel sheet. [0029] (12) A method for production of a steel sheet for hot stamping, wherein the steel 10 sheet for hot stamping, which is obtained by the production method according to any one of (7) to (9), is immersed in a hot-dip galvanizing bath, and then heated at a temperature of 600°C or lower to form an alloyed hot-dip galvanized layer on the surface of the steel sheet. [0030] 15 (13) A hot stamping steel material, wherein the hot stamping steel material has the chemical composition of: C: 0.18 to 0.26%; Si: more than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P: 0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less; 0: 0.0010 to 0.020%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and 20 impurities, in terms of % by mass, the concentration of a Mn-containing inclusion is not less than 0.010% by mass and less than 0.25% by mass, and the number ratio of a Mn oxide to the inclusion having a maximum length of 1.0 to 4.0 pm is 10.0% or more. [003 11 (14) The hot stamping steel material according to the above (13), wherein tlie 25 chemical composition includes one or more selected from the group consisting of Cr: 0.01 to 2.0%; Mo: 0.01 to 1.0%; V: 0.01 to 0.5%; W: 0.01 to 0.5%; Ni: 0.01 to 5.0%; and B: 0.0005 to 0.01%, in terms of % by mass. [0032] (15) The hot stamping steel material according to (13) or (14), wherein the chemical composition includes one or more selected from the group consisting of Ti: 0.001 5 to 0.5%; Nb: 0.001 to 0.5%; and Cu: 0.01 to 1.0%, in terms of % by mass. [Effects of the Invention] [0033] According to the present invention, good hydrogen embrittlement resistance can be secured even when processing leading to remaining of stress, such as punching, is 10 performed after hot stamping, and practice is easy, so that the range of applications (components) of the hot stamping method can be expanded. [Brief Description of the Drawings] [0034] [FIG. 11 FIG. 1 is a view illustrating a relationship between the amount of 15 diffusible hydrogen and the time until rupture. [FIG. 21 FIG. 2 is a view sliowing a hot stamping method and a die used in examples. [FIG. 31 FIG. 3 is a view showing an aspect of a constant load test piece used in examples. 20 [FIG. 41 FIG. 4 is a view showing an aspect of a steel sheet (member) pressed into a hat shape. [Modes for Carrying out the Invention] [0035] (1) Chemical Composition 25 The reason for specifying the chemical compositions of a steel slieet for hot stamping (hereinaftel; also refelred to as the "present invention steel sheet") and a hot stamping steel material (hereinafter, also referred to as the "present invention steel material") according to the present invention will be described. The " % in the following descriptions means "% by mass". [0036] 5 C is an element that is the most important in increasing the strength of a steel sheet by a hot stamping method. When the C content is less than 0.18%, it is difficult to secure a strength of 1500 MPa or more after hot stamping. Therefore, the C content is 0.1 8% or more. 10 On the other hand, when the C content is more than 0.26%, ductility after hot stamping becomes poor and it is difficult to secure a total elongation of 10% or more. Therefore, the C content is 0.26% or less. [0037] Mn is an element that is the most important in the present invention. Mn acts to enhance hydrogen embrittlement resistance by forming a Mn-containing inclusion in the steel. Remaining Mn that has not formed the inclusion acts to enhance hardenability. When the Mn content is less than 1.0%, it is difficult to ensure that the concentration of the 5 Mn-containing inclusion is 0.010% by mass or more. Therefore, the Mn content is 1 .O% or more. On the other hand, when the Mn content is more than 1.5%, the effect from the above-mentioned action is saturated, thus being economically disadvantageous, and mechanical characteristics may be deteriorated due to segregation of Mn. Therefore, the Mn content is 1.5% or less. 10 [0039] P is an element that is generally contained as an impurity. . When the P content is more than 0.03%, hot processability is significantly deteriorated. Therefore, the P content is 0.03% or less. The lower limit of the P content does not have to be particularly 15 specified, but is preferably 0.001% or more because excessive reduction causes a considerable burden on the steel-making process. [0040] S is an element that is generally contained as an impurity. When the S content is 20 more than 0.02%, hot processability is significantly deteriorated. Tlierefore, the S content is 0.02% or less. The lower limit of the S content does not have to be particularly specified, but is preferably 0.0005% or more because excessive reduction causes a considerable burden on the steel production process. [0041] 25 A1 is an element that acts to consolidate the steel by deoxidization. When the A1 content is less than 0.001%, it is difficult to perform sufficient deoxidization. Therefore, the A1 content is 0.001% or more. On the other hand, when the A1 content is more than 0.5%, generation of the Mn oxide is excessively suppressed, and it is difficult to secure the later-described Ivkoxide ratio, so that it is difficult to secure good hydrogen embrittlement 5 resistance. Therefore, the A1 content is 0.5% or less. [0042] N is an element that is generally contained as an impurity. When the N content is more than 0.1%, N is easily bound with Ti and B which are the later-described optional 10 elements to consume the elements, so that the effects of these elements are reduced. Therefore, the N content is 0.1% or less, preferably 0.01% or less. The lower limit of the N content does not have to be particularly specified, but is preferably 0.001% or more because excessive reduction causes a considerable burden on the steel-making step. [0043] 15 < 0 : 0.0010 to 0.020%> 0 forms a Mn oxide in the steel, which acts to enhance hydrogen embrittlement resistance by serving as a trap site for diffusible hydrogen and non-diffusible hydrogen. When the 0 content is less than 0.0010%, generation of the Mn oxide is not sufficiently accelerated, and the number ratio of the Mn oxide to the Mn-containing inclusion is less 20 than 10.0%, so that good hydrogen embrittlement resistance cannot be obtained with stability. Therefore, the 0 content is 0.0010% or more. On the other hand, when the 0 content is more than 0.020%, a coarse oxide is formed in the steel to degrade mechanical characteristics of the steel material. Therefore, the 0 content is 0.020% or less. [0044] 25 The present invention steel sheet and the present invention steel material have the above-described components as an essential component composition, and may fi~rther contain one or more of Cr, Mo, V, W, Ni, B, Ti, Nb and Cu as necessaly. [0045] , , , , and 5 These elements all act to enhance hardenability. Therefore, one or more of these elements may be contained. However, when B is contained in an amount exceeding the above-mentioned upper limit, hot processability is degraded and ductility is reduced. When Cr, Mo, W, V and Ni are contained in an amount exceeding the above-mentioned upper limit, the effect from the above-mentioned action is saturated, thus being 10 economically disadvantageous. Therefore, the upper limits of the contents of B, Cr, Mo, W, V and Ni are each as described above. For more reliably obtaining the effect from the above-mentioned action, it is preferred that the B content is 0.0005% or more, or the content of any of Cr, Mo, W, V and Ni elements is 0.01% or more. Ni acts to suppress degradation of the surface property of the hot-rolled steel sheet by Cu, and therefore it is 15 preferred that Ni is also contained when later-described Cu is contained. [0046] cTi: 0 to 0.5%>, and Ti, Nb and Cu all act to increase strength. Therefore, one or more of these elements may be contained. However, when the Ti content is more than 0.5%, generation 20 of the Mn oxide is excessively suppressed, and it is difficult to secure the later-described Mn oxide ratio, so that it is difficult to secure good hydrogen embrittlement resistance. Therefore, the Ti content is 0.5%. When the Nb content is more than 0.5%, controllability of hot rolling may be impaired. Therefore, the Nb content is 0.5% or less. When the Cu content is more than 1.0%, the surface property of the hot-rolled steel sheet 25 may be impaired. Therefore, the Cu content is 1.0% or less. For obtaining the effect from the above-mentioned action more reliably, it is preferred that any of Ti (0.001% or more), Nb (0.001% or more) and Cu (0.01% or more) is contained. Since Ti is preferentially bound with N in the steel to form a nitride, and thereby inhibits B from being wastefully consumed by forming a nitride, so that the effect by B can be further increased, it is preferred that Ti is also contained when the above-mentioned B is contained. 5 [0047] The balance includes Fe and impurities. [0048] (2) Inclusion Next, the reason for specifying the concentration of the Mn-containing inclusion 10 and the number ratio of the Mn oxide to the Mn-containing inclusion having a maximum length of 1.0 to 4.0 pm in the present invention steel sheet and the present invention steel material will be described. [0049] The Mn-containing inclusion plays an important role in suppression of hydrogen embrittlement together with the number ratio of the Mn oxide to the later-described Mn-containing inclusion having a maximum length of 1.0 to 4.0 pm. When the concentration of the Mn-containing inclusion is less than 0.010%, it is difficult to obtain 20 good hydrogen embrittlement resistance. Therefore, the concentration of the Mn-containing inclusion is 0.010% or more. On the other hand, when the concentration of the Mn-containing inclusion is 0.25% or more, toughness may be reduced. Therefore, the concentration of the Mn-containing inclusion is less than 0.25%. [0050] 25 The concentration of the Mn-containing inclusion is determined in accordance with the following procedure. That is, a steel sheet is electrolyzed at a constant current in an electrolytic solution with acetylacetone and tetramethylammonium dissolved in methanol, a filter having a pore diameter of 0.2 pm is used to collect residues, the mass of the residues is divided by an electrolysis amount (mass of the steel sheet lost by electrolysis), and the obtained value is multiplied by 100 to be described in terms of a 5 percentage. It is confilmed that the inclusion extracted by the electrolysis metliod contains Mn by EDS (energy dispersive X-ray spectroscopy) with a SEM (scanning electron microscope). [0051]