HIGH STRENGTH STEEL SHEET EXCELLENT IN DELAYED FRACTURE RESISTANCE AND LOW TEMPERATURE TOUGHNESS, AND HIGH STRENGTH MEMBER MANUFACTURED USING THE SAME [Technical Field] [0001] The present invention relates to high-strength steel sheets suitable for hot stamping, for example, and more particularly relates to a high-strength steel sheet excellent in delayed fracture resistance and low-temperature toughness. This application claims the benefit of priority from Japanese Patent Application No. 2013-051953, filed on March 14, 2013, the content of which is incorporated herein by reference. [Background Ait] [0002] Vigorous attempts are made in general to reduce the mass of a vehicle, including an automobile, by using high strength materials in the transportation equipment industry. In the automobile industry, however, the mass of a vehicle has tended to increase recently by adding more crash-safety capabilities and other new functions. The use of the high strength steel sheets has steadily increased in an effort to offset even a part of the increase in the automobile body mass as well as to improve efficiency in fliel consumption and to reduce carbon dioxide emission. [0003] In the tendency of the increasing use of the high-strength steel sheets, what comes to the surface as a large obstacle is a phenomenon called "deterioration in shape fixability" that inevitably occurs when the strength of the steel sheet is increased. This phenomenon generally refers to the fact that it becomes difficult to obtain a desired shape due to springback after shaping. The amount of the springback increases as the steel sheet becomes stronger. To solve this problem, efforts have been made to change product shapes or to add a manufacturing process (for example, a restriking process) that has not been necessary in the case of low-strength materials (that are excellent or non-problematic in terms of the shape fixability). 5 [0004] As another solution to this problem, a hot shaping method called hot stamping has come to people's attention. With the hot stamping, a steel sheet (a material to be shaped) is heated to a predetermined temperature (generally for an austenite phase) and the strength is made lower (i.e., easier for shaping). The steel sheet is then shaped with dies of a 10 lower temperature (for example, at room temperature) than that of the material to be shaped. In this way, the material is shaped easily and cooled rapidly (quenching) using this temperature difference, so that the product strength after shaping is secured. [0005] The hot stamping has been widely recognized to be useful in recent years and the 15 possibilities of application to a wide range of steel members have been studied. Among such steel members, some are exposed to a harsh environment in terms of corrosion resistance, such as underbody parts of an automobile, or some have pierced portions for allowing optional parts to be attached. For such steel members, delayed fracture resistance as well as product strength as parts has been regarded as one of the important 20 required properties. [0006] The delayed fracture becomes a concern in a condition that hydrogen penetration is expected to be larger due to exposure to a corrosive environment and/or residual stress remaining in a pierced portion becomes significant to a degree that is not negligible. 25 [0007] Generally, the delayed fracture resistance of a steel sheet is known to be deteriorated as the strength of the steel sheet increases. Considerations have already been made in advance to some of the steel sheets (products) having a strength increased by hot stamping. [0008] For example, Patent Literature 1 discloses a technique related to the steel sheet having properties to restrain delayed fractures. For this purpose, one or more of oxides, sulfides, and composite crystals and composite precipitates of Mg, each having an average grain size within a predetermined range, are added at a predetermined density in the steel sheet. [0009] Patent Literature 2 discloses a technique for improving delayed fracture resistance by carrying out punching perforation (piercing) at a high temperature (hot working) after heating for hot stamping and before pressing, and thus improving punching quality. [0010] Patent Literature 3 discloses a technique for obtaining a steel sheet with an excellent delayed fracture resistance by including Fe-Mn composite oxides having predetermined dimensions at a predetermined density in the steel sheet. [0011] The technique disclosed in Patent Literature 1, however, is not very easy even for those skilled in the art to carry out. Mg, which is not easy to be included in the steel, is required to exist in the steel and the product containing Mg needs to be highly controlled. [0012] The technique disclosed in Patent Literature 2 presupposes that the piercing work is performed during hot working. The effect is not clear, however, in the case that the piercing work is carried out after hot stamping, that is, the residual stress may remain to a degree that is not negligible. [0013] On the other hand, the technique disclosed in Patent Literature 3 is excellent since effective hydrogen trapping sites are formed in the steel by combining deoxidation control during the steel making process with appropriate rolling conditions, and thus a certain 5 positive effect may be expected. [0014] The present inventors carried out experiments repeatedly to investigate the mechanical properties, after hot pressing, of the steel sheets prepared with reference to Patent Literature 3. The experiment results showed a certain positive effect in the delayed 10 fracture resistance. However, there were found problems that low-temperature tougluiess was not sufficient and casting defects occurred frequently especially in high C (carbon) concentration cases in which the cast material was not able to be rolled or, if Tollable, the product yield rate dropped considerably. [0015J 15 As described above, the facts are that there is not found a technique that can easily provide steel members, at a satisfactory yield rate, that are processed after hot stamping in a way that the residual stress remains, such as in piercing work, and that have the delayed fracture resistance and the low-temperature toughness at the same time. [Prior Art Literature(s)] 20 [Patent Literatures] . [0016] [Patent Literature 1] JP 2006-9116A [Patent Literature 2] JP 2010-174291A [Patent Literature 3] WO 2012/120692 25 [Summary of Invention] [Problems to Be Solved by the Invention] [0017] In view of the foregoing, an object of the present invention is to provide a steel sheet for hot stamping that can be easily manufactured with an existing steel making facility at a high yield rate and that has an excellent delayed fracture resistance and low-temperature toughness even if the steel member is processed in the way that the residual stress is expected to remain, as is in the piercing work, after being shaped in a hot stamping facility. [Means for Solving the Problems] [0018] The present inventors have eagerly studied steel sheets for hot stamping in order to solve the aforementioned problem. The present inventors adopted a base technique that allows Mn oxide, which can be easily generated in the steel, to be included in the steel sheets, and further studied problems that could not be solved with the base technique alone. [0019] In particular, the following two problems are to be solved: (A) The steel sheet having a tensile strength of 1500 MPa or more obtained after hot stamping needs to contain carbon at 0.2% or more (in mass%, hereinafter % is expressed by mass). In the carbon steel having a high concentration of carbon, however, the carbon itself has a deoxidation capability, and the deoxidation reactions generate CO, which forms bubbles in molten steel. The bubbles remain in casting products and tend to cause casting defects. [0020] (B) The other problem that needs to be seriously taken into account is that although it is effective to disperse oxides in the steel sheet as hydrogen trapping sites, the dispersion of the oxides may deteriorate low-temperature toughness depending on the forms and densities of the oxides. [0021] In order to address (A), the present inventors have studied a method to restrain bonding of carbon with oxygen by adding a small amount of Si and found that the suitable concentration of Si exists within a range that was described not suitable in Patent Literature 5 3. The present inventors have also found that the forms of inclusions (oxides) can be transformed by adding Si at a predetermined concentration, which is also effective to address (B). [0022] Now, the present invention, which has been completed through the 10 aforementioned studies, will be summarized as below. [0023] 0) A high strength steel sheet containing chemical components of, in mass%, C: 0.20 to 0.42%, 15 Si: 0.06 to 0.5%, Mn: 0.2 to 2.2%, Cr:0.1 to 2.5%, B: 0.0005 to 0.01%, O: 0.0020 to 0.020%, 20 Al: 0.001 to 0.03%, Ti: 0.001 to 0.05%, Nb:0to0.1%, Mo: 0 to 1.0%, W: 0 to 0.5%, 25 V:0to0.5%, Ni:0to5.0%, Cu:Otol.O%, N: 0.1% or less, P: 0.03% or less, S: 0.02% or less, and 5 the balance: Fe and inevitable impurities. In steel, 5 x 10 pieces per mm or more to 1 x 10 pieces per mm or less of Mn oxides having a maximum length of 1 urn or more to 5 um or less are present, and 1.7 x 10 pieces per mm or more to 5 x 10 pieces per mm or less of Mn-Si composite oxides having a short-axial length of 1 um or more and a longitudinal length of 10 10 um or less are present. [0024] (2) The high strength steel sheet according to (1), ftirther containing one or more selected from the group consisting of, in mass%, Nb: 0.01 to 0.1%, 15 Mo: 0.01 to 1.0%, W: 0.01 to 0.5%, V: 0.01 to 0.5%, Ni: 0.01 to 5.0%, and Cu: 0.01 to 1.0%. 20 [0025] (3) The high strength steel sheet according to (1) or (2), in which the steel sheet is cold rolled by 35 to 70%. [0026] 25 (4) The high strength steel sheet according to (3), in which the steel sheet is plated. [0027] (5) The high strength steel sheet according to (4), in which the steel sheet has an hot-dip-plated Al layer on the surface thereof, and an Fe-Al-Mn-Cr-B alloy layer having a 5 thickness of 1 um or more and 10,um or less is present at an interface between the Al layer and the steel sheet. [0028] (6) A high strength member, in which the high strength steel sheet according to (3) is 10 heated to a temperature at wliich at least a portion of the steel sheet is transformed into an austenite phase, and is subsequently stamped with dies such that shaping and quenching are carried out in one process. [0029] (?) 15 A high strength member, iti which the high strength steel sheet according to (4) or (5) is heated to a temperature at which at least a portion of the steel sheet is transformed into an austenite phase, and is subsequently stamped with dies such that shaping and quenching are carried out in one process. [Effects of the Invention] 20 [0030] A steel sheet of the present invention can be manufactured with an existing steel making facility. Even if the steel sheet, which is shaped in a hot stamping facility, undergoes processing, such as a piercing work, in which residual stress is expected to remain, the steel sheet still has an excellent delayed fracture resistance and 25 low-temperature toughness. Consequently, the steel sheet of the present invention has an effect to expand the application range (the range of parts to be applied) of hot stamping. 9 [Brief Description of the Drawing(s)] [0031] [FIG. 1] FIG. 1 schematically illustrates the maximum length of oxides in a steel sheet, in which (a) is a rectangular-shaped oxide and (b) is a polygonal oxide. 5 [FIG. 2] FIG. 2 schematically illustrates lengths in the short axis direction and in the longitudinal direction of a drawn oxide. [FIG. 3] FIG. 3 shows photographs for exemplifying variations of blowhole defects depending on different Si contents. [FIG. 4] FIG. 4 illustrates a relation between cold-rolling reduction ratio (%) and 10 ductile-brittle transition temperature (°C). [FIG. 5] FIG. 5 schematically illustrates a hat shape in which the figures are approximate dimensions expressed in mm. [Mode(s) for Carrying out the Invention] [0032] 15 Now, the present invention will be described in detail. [0033] Reasons for imposing limitations on the chemical components of a steel sheet of the present invention will be described first, where "%" expresses mass%. [0034] 20 C is the most important element in increasing the strength of a steel sheet by hot stamping. C needs to be contained in the steel sheet at 0.20% or more in order to obtain a strength of at least about 1500 MPa. If the C concentration exceeds 0.42%, on the other hand, the steel sheet cannot secure low-temperature toughness even if the oxide control 25 according to the present invention is employed. Accordingly, the maximum concentration of C is set to 0.42%. For a more excellent low-temperature toughness, 10 0.36% or less is preferable. [0035] Si contributes to restraining the generation of CO bubbles, from C deoxidation, 5 which cause casting defects. The present inventors performed experiments of melting and casting the steel that contains, as major compositions, C: 0.20 to 0.42%, Mn: 1 to 1.5%, Cr: 0.2%, B: 0.0020% and the balance: Fe, in order to obtain the steel in which Mn oxides were dispersed. Results showed that bubble-like defects were sometimes generated on the surface of casting products. The results also showed that the bubble-like defects were 10 sporadically generated from a C concentration of around 0.25% and frequently generated from 0.3% or above. There existed deep defects as well in some casting products that, the inventors had to determine, were not appropriate for rolling even if the surface is treated. [0036] To solve this problem, the inventors attempted to add a small amount of Si (for Si 15 deoxidation). Si is an element that has a better deoxidation capability than Mn and a less aggregation tendency of resulted oxides than Al. As a result, the inventors found that a Si content of 0.06% or more completely suppressed blowhole defect generation. Accordingly, the minimum concentration of Si is set to 0.06%. For the purpose of blowhole defect suppression, there is no limit in terms of the upper boundary. Too much 20 oxide generation, however, causes deterioration in the low-temperature toughness. Accordingly, a permissible upper limit is 0.5%, preferably 0.3% or" less, and more preferably 0.2% or less. [0037] The amount of Si is a total sum of Si that forms oxides (including Si-Mn 25 composite oxides) and Si that is solved in the steel without bonding with oxygen. Si acts as the deoxidation element to restrain the generation of CO bubbles as described above. 11 Moreover, the inventors have found that Si forms composite oxides with Mn and the oxides contribute effectively to improving the low-temperature toughness, which should be also included in the gist of the present invention. [0038] 5 Mn is the most important element for the present invention. Mn oxides are quite important not only in functioning as hydrogen trapping sites but in serving to form composite oxides with Si. The composite oxides with Si are extremely important in securing the low-temperature toughness. This effect appears when the Mn concentration 10 is 0.2% or more. However, this effect becomes saturated and a risk of deteriorating mechanical properties becomes larger due to solidifying segregation when the Mn concentration goes beyond 2.2%. Therefore, the upper limit of the Mn concentration is set to 2.2%, preferably 2.0%, and more preferably 1.8%. [0039] 15 Cr has an effect to improve hardenability and thus is contained in the steel sheet. The effect becomes apparent at 0.1% or more. The effect, however, becomes saturated at a concentration exceeding 2.5% and thus the upper limit is set to 2.5%, preferably 2.0%, and more preferably 1.8%. 20 [0040] As in Cr, B has an effect to improve hardenability and thus is contained in the steel sheet. The effect becomes apparent at 0.0005% or more. An excess concentration deteriorates workability in hot working and ductility, and thus the upper limit is set to 25 0.01%. To flirther improve hardenability and restrain deterioration in the workability in hot working and deterioration in the ductility, the B concentration is preferably from 12 0.0010 to 0.007%. [0041J <0: 0.0020 to 0.020%> O is an indispensable element to form oxides. Oxides are extremely important in 5 forming hydrogen trapping sites and in affecting the low-temperature toughness, and thus must be controlled properly. When the concentration is less than 0.0020%, the density of the oxides is not enough. If more than 0.02%, the coarsening of the oxides may lead to deterioration in mechanical properties. Therefore, oxygen limits are set to the range described above. 10 [0042] Al is a strong deoxidation element and must be controlled carefully. Containing Al at a concentration exceeding 0.03% makes it difficult to secure predetermined amounts of Mn oxide that is effective for delayed fracture resistance and Mn-Si composite oxide 15 that is important to secure low-temperature toughness. Accordingly, the upper limit is set to 0.03% and preferably 0.01%. The lower limit is set to 0.001% because any concentration less than 0.001% will impose too much burden on the steel making process. [0043] 20 Ti is an element that has a capability of deoxidation and has an impact on the forming of Mn oxides and Mn-Si composite oxides. r.Ti must be controlled to a concentration of 0.05% or less and preferably 0.03% or less. The lower limit is set to 0.001% because any concentration less than 0.001% will impose too much burden on the steel making process. 25 [0044] Now, components to be selectively added will be explained as below. 13 Nb may be appropriately contained because adding Nb has an effect to miniaturize ciystal grains and improve toughness. The effect appears at 0.01% or more. It is desirable to set the lower limit to 0.01% to obtain such effect. The upper limit is set 5 to 0.1% because the effect is saturated at a concentration exceeding 0.1%. [0045] One or more of , , and Each of these elements has an effect to improve hardenability and may be contained appropriately. The effect becomes apparent for each element at 0.01% or more. 10 Each one is a high-priced element and thus the upper limit is preferably set to a concentration at which the effect becomes saturated, i.e., 1.0% for Mo, and 0.5% for W and V. [0046] 15 Ni is an element that has an effect to improve hardenability and is to be utilized effectively. The effect becomes apparent at 0.01% or more. Ni is a high-priced element and the upper limit is set to a concentration at which the effect becomes saturated, i.e., 5.0%. It is desirable to include Ni together with Cu because Ni has an effect to restrain degradation of the surface quality of a hot-rolled steel sheet caused by adding Cu as 20 described below. . [0047] Cu has an effect to increase the strength of the steel sheet by an addition of 0.01% or more. Too much addition, however, leads to degradation of the surface quality of a 25 hot-rolled steel sheet. The upper limit is set to 1.0% accordingly. [0048] 14 The remaining component in the present invention other than aforementioned elements is Fe. Inevitable impurities derived from melted materials such as scraps and from refractory materials are allowed to be included. Typical impurities are listed below. [0049] 5 N is bonded to Ti and B easily and needs to be controlled to a level of a concentration that will not reduce the effects expected for Ti and B. The permissible level of the concentration is 0.1 % or less and preferably 0.01 % or less. It is desirable to set the lower limit to 0.0010% because any concentration less than a necessary level will impose 10 too much burden on the steel making process. [0050] P, which is contained as an impurity, has a negative impact on workability in hot working and thus must be limited to 0,03% or less. It is preferable to limit P as low as 15 possible. It is desirable, however, to set the. lower limit to 0.001% because any concentration less than a necessary level will impose too much burden on the steel making process. [0051] 20 S, which is contained as an impurity, has a negative impact on mechanical properties including workability in hot working, ductility, and toughness, and thus must be limited to 0.02% or less. It is preferable to limit S as low as possible. It is desirable, however, to set the lower limit to 0.0001% because any concentration less than a necessary level will impose too much burden on the steel making process. 25 [0052] Described now will be reasons to impose limitations on Mn oxide and Mn-Si 15 composite oxide. [0053] Regarding Mn oxide, the oxide itself and a void formed around the oxide during cold rolling become a hydrogen trapping site in the steel sheet, whereby an excellent 5 delayed fracture resistance is developed. Consequently, the Mn oxide needs to be dispersed at a predetermined density. As will be shown in Examples, the effect does not appear clearly if the oxides are less than 5^10 pieces per mm . On the other hand, from a view point of the delayed fracture resistance, the upper limit is not necessarily set on the density. The upper limit of the density, however, is set to 1 x 105 pieces per mm2 in order 10 to avoid a negative impact on mechanical properties including ductility and toughness. [0054] To identify a Mn oxide, energy dispersive X-ray spectroscopy (EDS) analysis using a scanning electron microscope (SEM) was employed. An object from which Mn and O (oxygen) were detected at the same time was regarded as a Mn oxide. If the 15 maximum length of an object to be analyzed is less than 1 um, sufficient analytical accuracy is not secured. Accordingly, the lower limit of the maximum length of the Mn oxide was set to 1 um or larger. From a view point of the delayed fracture resistance, the upper limit is not necessarily set on the size of the oxide. The upper limit of the size, however, is set to 5 um or less in order to avoid a negative impact on mechanical 20 properties including ductility and tougliness. With reference to FIG. 1 that schematically illustrates shapes of oxides, a maximum length 3 of the oxide as used herein is designated by the longest diagonal of a rectangular-shaped oxide 1 or a polygon oxide 2. [0055] According to the present inventors' study, Mn oxide is shaped as either a rectangle 25 or a polygon and maintains the shape after cold rolling. In contrast, Mn-Si composite oxide has a drawing capability with cold rolling. The dispersion of moderately drawn 16 Mn-Si composite oxides in the steel is likely to contribute to securing the low-temperature toughness. As used herein, the term a drawn Mn-Si composite oxide refers to the one having a longitudinal length that is approximately 3 times or more longer than the short-axial length. Inclusions dispersed in the steel sheet were investigated by the EDS 5 analysis using SEM. The inclusions from which Mn, Si, and O (oxygen) were detected at the same time were regarded as Mn-Si composite oxides. For reliable analysis, the dimension of an object needs to have a short-axial length of 1 urn or more. For this reason, the lower limit of the size of a Mn-Si composite oxide was set to a short-axial length of 1 um. From a view point of securing analytical reliability, the upper limit is not 10 necessarily set on the short-axial length. The upper limit of the short-axial length, however, is set to 3 um, preferably 2 um, to avoid the deterioration in mechanical properties (such as elongation and toughness). For the purpose of securing toughness, the upper limit is not necessarily set on the longitudinal length. The upper limit of the longitudinal length, however, is set to 10 pm, preferably 5 pm, to avoid deterioration in 15 ductility that occurs if the size is too large (too long). As schematically illustrated in FIG. 2, a short-axial length 5 and a longitudinal length 6 as used herein respectively refer to the length in the short axis direction (short-axial length) 5 and the length in the longitudinal direction (longitudinal length) 6 of a drawn Mn-Si composite oxide 4. [0056] 20 As will be shown in Examples, 1.7 x 10 pieces per mm of Mn-Si composite oxides or more need to be contained.' A density less than this number does not secure an . ' excellent low-temperature toughness. On the other hand, if more than 5 x 103 pieces per mm of Mn-Si composite oxides exist, the ductility in the direction perpendicular to a rolling direction deteriorates considerably, and thus the upper limit is set to 5 x 103 pieces 25, per mm2. [0057] 17 A procedure to obtain the density of oxides is as follows: the number of the target oxides within the fields of view was counted (totaled) using SEM with 3000 magnifications and 10 fields of view. One field is dimensioned about 40 pm by 30 um. The number of oxides in an area of 1.2 x 104 um2 (i.e., 40 x 30 x 10) was converted into a 5 density per square millimeter (mm2). [0058] A method for producing a steel sheet of the present invention will now be described. [0059] 10 A steel sheet of the present invention is produced by carrying out steelmaking, casting, hot rolling, cold rolling, and annealing based on conventional procedures. Plating may be carried out as well. Steel can be prepared and casted in line with the current conditions of a manufacturer in terms of materials to be used (concentration of impurities), yield rate of each element and so forth. For example, steel is made from 15 materials excluding Si by a normal procedure, and Si is added to the steel, and then the steel is casted after a predetermined period of time. [0060] As will be explained in Examples, the density of Mn oxides decreases as the elapsed time between Si addition and steel casting becomes longer. In contrast, the 20 density of Mn-Si composite oxides increases as the elapsed time between the Si addition and the steel casting becomes, longer. An appropriate range of the elapsed time between the Si addition and the steel casting was 35 to 145 seconds with a small-scale melting furnace that the present inventors employed. The time, however, can be set in line with the current conditions of a facility to be used. From a productivity point of view, 25 continuous casting is desirable. [0061] 18 For hot rolling, for example, the following parameters may be used: a temperature of 1200 to 1250°C for heating a casting product, a reduction ratio of 50 to 90% for rough rolling, a reduction ratio of 60 to 95% for finish rolling, and a finishing temperature of about 900°C. 5 [0062] A cold-rolling reduction ratio in cold rolling is very important, which must be 35% to 70%. Rolling with a reduction ratio of 35% or more is required to create a void around a Mn oxide and to draw and deform a Mn-Si composite oxide appropriately. However, if the reduction ratio is too high, the once-generated void around the Mn oxide is 10 crushed and disappears. Moreover, the drawn and deformed Mn-Si composite oxide is split and ceases to contribute to the toughness. The upper limit therefore needs to be set to 70%. [0063] It is desirable to set a temperature for annealing a cold-rolled steel sheet to 700 to 15 850°C. The temperature, however, may be set to less than 700°C or to more than 850°C in order to distinguish a product by having a unique mechanical property. From a productivity point of view, continuous annealing is desirable. [0064] The steel sheet may be a steel sheet plated with Al, Zn, or the like after annealing. 20 From a productivity point of view, it is desirable to carry out annealing and plating as one continuous operation. A Zn-plated steel sheet may be heated to transform, the plating layer into Fe-Zn alloy. In the case of an Al-plated steel sheet, a Fe-AI-Mn-Cr-B alloy layer is formed at the interface between the Al plating layer and the steel sheet base. For the Al-plated steel sheet, the transfer of hydrogen from the Al plating layer to the steel 25 sheet base is restrained because the Fe-Al-Mn-Cr-B alloy layer is formed at the interface between the Al plating layer and the steel sheet base. Incidentally, almost no Si is 19 contained in this alloy layer of the steel sheet according to the present invention. This is because most of the added Si is consumed to reduce Mn oxides to Mn-Si composite oxides under such oxygen concentration state that Mn oxides and Mn-Si composite oxides are generated as in the present invention, and thus almost no Si is contained in the 5 Fe-Al-Mn-Cr-B alloy layer at the interface between the Al plating layer and the steel sheet base. [0065] The annealed and plated steel sheet (hoop) may go through skin pass rolling and roll leveller treatment The resulted strain is preferably controlled to 10% or less. 10 [0066] The produced steel sheet according to the present invention is made into a high strength member by hot stamping in which shaping and quenching are carried out as one process, for example. In the production of the high strength member, a steel sheet (blank), which has been cut to predetermined dimensions according to requirements, is heated and 15 then stamped with dies. For heating, a method such as furnace heating, electric heating, induction heating and so forth may be employed. The temperature to which the whole blank is heated is generally set to a level for an austenite phase. Only a portion of the blank may be heated to the level for the austenite phase in order to add a unique property to the member. Cooling by the dies is generally carried out at a cooling rate that allows the 20 portion that has been heated to the level for the austenite phase to be transformed into a inartensite phase. 'In order to add a unique property to the member, however, the cooling rate may be set slower so that the portion that has been heated to the level for the austenite phase is not transformed into the martensite phase. [0067] 25 Delayed fracture resistance of the steel sheet was evaluated by conducting piercing tests with different clearances and by observing presence or absence of crack 20 generation on a wall portion of each pierced hole. [0068] More particularly, a hole of 10 mmcp was pierced through a steel sheet oft (mm) in thickness. The diameter of a punch Dp was constantly set to 10 mm. The inner 5 diameter of a die Di was varied in a clearance range of 5 to 30% where the clearance was calculated from (Di - Dp) / 2t x 100. Subsequently, the presence or absence of cracks in the hole wall portion was obsei'ved. The steel sheet in which no crack generation was observed was determined excellent in the delayed fracture resistance. The number of pierced holes for each clearance type was 5 or more and all the hole walls were observed. 10 [0069] Toughness was evaluated using the Charpy impact test in accordance with JIS Z 2242. A test piece was prepared in accordance with No. 4 test piece specified in JIS Z 2202. The thickness of the test piece was the same as that of a sample as it was. [0070] 15 The tests were conducted in a range from -120°C to 20°C and a ductile-brittle transition temperature was determined based on changes in absorbed energy. [Example(s)] [Example 1] [0071] 20 Experiments of melting and casting raw materials using a small-scale melting furnace were' conducted. The chemical components were adjusted so as to contain C: 0.36%, Mn: 1.3%, P: 0.02%, S: 0.004%, Cr: 0.2%, B: 0.0025%, Ti: 0.01%, Ah 0.002%, N: 0.003%, O: 0.0150%, and the balance: Fe and inevitable impurities. [0072] 25 A predetermined amount of Si was subsequently added to the melted materials, which were casted into a mold having inner dimensions (in millimeter) of 110 x 220 x 400 21 (maximum height) 90 seconds after Si addition. The amount of added Si ranged from 0 (no addition) to 0.3%. The concentration was calculated with the assumption that all of the added Si remained in the casting product. Solidified surfaces (two faces) of the casting product having dimensions of 220 by 400 were ground smooth by 5 mm and the 5 generation of defects derived from bubbles was investigated. The casting product was subsequently hot rolled from 110 mm to 30 mm. The hot-rolled material was analyzed to determine the concentration of major components. The results are summarized in Table 1. [0073] 10 [Table 1] [0074] A large number of blowhole defects were observed in No. 1 to which Si had not been added (the concentration was less than 0.001%) and in No. 2 to which 0.02% Si had 5 been added (the concentration was 0.020%). In contrast, no blowhole defect was observed in Nos, 3, 4, and 5 in each of which a Si content was 0.06% or more. The 23 surface conditions of No. 2 and No. 3 are shown for contrast in FIG. 3(a) and FIG. 3(b), respectively. [0075] No. 1 and No. 2 started to develop cracking during rolling, which made it difficult to roll to less than 30 mm. In contrast, no problem was found in the roUability of No. 3 to No. 5, which were subsequently rolled down to 2 mm without problem (this rolling corresponds to finishing rolling). It became apparent that casting defects were able to be suppressed completely in the same way for other C concentration cases than this Example if Si is added to 0,06% or more. [Example 2] [0076] Raw materials were melted using a small-scale melting furnace and the chemical components were adjusted so as to contain: C: 0.3%, Mn: 1.3%, P: 0.02%, S: 0.004%, Cr: 0.2%, B: 0.0020%, Ti: 0.01%, Al: 0.002%, N: 0.004%, O: 0.0150%, and the balance: Fe and inevitable impurities. [0077] Si was subsequently added to molten steel to a concentration of 0.15%. The molten steel was charged into a mold, which was repeated 5 times at intervals of 30 seconds after the Si addition. [0078] Each casting product obtained was heated to 1250°C and hot rolled into a 2.8 mm thick hot-rolled steel sheet with a finishing temperature of 900°C and a coiling temperature of 600°C. The hot-rolled steel sheet was cold rolled after pickling and a 1.4 mm thick cold-rolled steel sheet was obtained. [0079] The results of analysis for chemical components of the cold-rolled steel sheets are 24 shown in Table 2. Each of the steel sheets having numerical references of 2a-1, 2a-2, 2a-3, 2a-4, and 2a-5 falls within the component range of the present invention. [0080] The cold-rolled steel sheets were immersed in a salt bath and annealed at 800°C for 1 minute and steel sheets for hot stamping were obtained. [0081] Hot stamping was carried out with the following procedure: each steel sheet for hot stamping was kept at 900°C for 5 minutes and then immediately pressed and held for 30 seconds by a pair of water-cooled upper and lower plate dies. [0082] A cross section of each hot stamped sample (hereinafter referred to as a HS sample), which was taken parallel to a cold rolling direction, was observed using SEM. The size and the density of both Mn oxides and Mn-Si composite oxides were measured by the aforementioned methods. [0083] Vickers hardness of the cross section of each HS sample parallel to the cold-rolling direction was also measured. The Vickers hardness was measured at a total of 10 points including 5 points located at one fourth of the thickness and 5 points at three fourths of the thickness from one surface of the HS sample. The 10 measurements were averaged to obtain a cross sectional surface hardness. The test load applied to the indenter was 1 kgf. [0084] A 100mm by 100mm test piece for evaluation of delayed fracture resistance and a test piece for the Charpy test were taken from each of the HS samples. [0085] A hole of 10 mmtp was pierced with the center of the hole being aligned with the 25 intersection of the diagonals of the 100 mm by 100mm. test piece. The clearances were 8.9% (10.25 mm), and 28.6% (10.80 mm