Technical field
[0001]　The present invention relates to a hot stamping molded article and a manufacturing method thereof.
BACKGROUND
[0002]　Conventionally, in view of global environmental issues and collision safety performance, thinner and higher strength of automobile structural parts are required. To meet these demands, automotive structural parts and materials of high strength steel sheet has increased. Further, as a forming method of the high strength steel sheet, there is known a method called hot stamping. The hot stamp, and pressed at C content is high temperature range of not lower than 700 ° C. The steel plate of about 0.20 wt% to 0.22 wt%, perform quenching in a press-mold or press die out. According to hot stamping, for performing the molding in a high temperature range in which the strength of the steel sheet is reduced, it is possible to suppress molding defects such as occurs in cold pressing. Also, since the tissue to the martensite as a main phase by quenching after molding can be obtained, it is possible to obtain a high strength. Therefore, tensile strength hot stamping of about 1500MPa are used worldwide extensively.
[0003]However, the present inventors have conducted research for further strengthening, a hot stamp molded product having a tensile strength of at least 1900 MPa, it was revealed that there is the low stress fracture occurs. When hot stamping molded low stress fracture occurs is used structural parts for automobiles, there is a possibility that the part even when an impact that was calculated to withstand the design stage are destroyed. Thus, inhibition of the low stress fracture is critical to ensuring collision safety of automobile structural parts. So far, the low stress fracture of maraging steel is known, low-stress destruction of the hot stamping body is not known.
CITATION
Patent Document
[0004]
Patent Document 1: JP 2012-41613 JP
Patent Document 2: JP 2014-156653 Patent Publication
Patent Document 3: Japanese Patent No. 5756773
Patent Document 4: JP 2014-118613 Patent Publication
Patent Document 5: Japanese Patent No. 5402191 Publication
Non-patent literature
[0005]
Non-Patent Document 1: River portion Yoshikuni: Iron and Steel, 68, (1982), 2595
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　The present invention aims at providing a hot stamping molded article and a manufacturing method thereof which can suppress the low stress fracture with high strength.
Means for Solving the Problems
[0007]　The present inventors have conducted studies to elucidate the cause of low stress fracture occurs in hot stamping molded article having a tensile strength of at least 1900 MPa.
[0008]　The following describes an indicator for low stress fracture in the present application. In the present application, using a tensile test specimen conforming to JIS Z 2201, when subjected to a tensile test under the conditions conforming to JIS Z 2241, a low stress fracture results in a material that occurs before the break is satisfied the following formula 1 It called the material, that the material of low stress fracture does not occur and the resulting material after the formula 1 is satisfied. In Formula 1, sigma represents a true stress, epsilon true strain.
　　dσ / dε = σ ··· (Equation 1)
[0009]
　Equation 1 is the load maximum condition derived from the constant volume law during deformation. Usually, tensile immediately after the start of the study dσ / dε is greater than sigma, sigma increases with dσ / dε decreases As the deformation progresses. Then, the material low stress fracture does not occur, the load is maximized at the moment of dσ / dε is equal to the sigma, since thereafter occurs constriction tensile specimens, the load is reduced. On the other hand, in the material low stress fracture occurs before the constriction tensile test piece occurs, i.e. dσ / dε is breaking with a larger stage σ occurs.
[0010]
　The present inventors have found that, when the study was first to investigate the relationship between the organization and the low stress fracture of hot stamping body. As a result, as the old γ grains are fine and the higher coarse carbides is small, it was found that low stress fracture hardly occurs.
[0011]　However, in the conventional hot stamp, to achieve both a reduction in the old γ grain refinement and coarse carbide is difficult, it is impossible to sufficiently improve the fracture characteristics by suppressing low stress fracture. In other words, the refinement of the prior γ grain, but lowering of the heating temperature and heating time of the hot stamping is preferred, reduction of the heating temperature and the heating time leads to a reduction in the amount of dissolution of carbides during heating, coarse carbides remain It becomes easier. Conversely, the reduction of the coarse carbides, an increase of the heating temperature and heating time of the hot stamping is preferred, an increase of the heating temperature and heating time leads to coarsening of the old γ grains.
[0012]　Accordingly, the present inventors have, to achieve both a reduction in the old γ grain refinement and coarse carbides hot stamping molded article was examined for an improved organization of the steel sheet subjected to hot stamping. As a result, in order to suppress residual coarse carbides, the fresh martensite and tempered martensite and the main phase, it is preferable to reduce the easy ferrite and pearlite containing coarse carbides, and, during the heating of the hot stamp to obtain a fine gamma, to keep finely dispersing the carbide serving as the reverse transformation of nucleation sites in the steel sheet to gamma it is preferable revealed. The steel sheet having such a structure by hot stamping, excellent hot stamping body very fracture characteristics. However, such steel plate has the following problems.
[0013]　Main phase hardness of the steel sheet of the fresh martensite and tempered martensite, the hardness after hot stamping, that is, hardness and comparable hot stamping molding. Since tensile strength Vickers hardness of the hot-stamping of 1900 MPa is about 550 Hv, if the tensile strength is to be obtained more hot stamping member 1900 MPa, the Vickers hardness of the steel sheet becomes more than about 550 Hv. When producing the hot stamping material, when forming the blank by blanking by shear cutting or stamping or the like steel plate before hot stamping, blanking or more steel Vickers hardness 550Hv is extremely difficult.
[0014]
　Therefore, the present inventors have conducted further extensive studies. As a result, the present inventors have found that performing at least two hardened under a predetermined condition after blanking, have a new tissue, they found that the hot stamping molded article can be obtained with excellent fracture properties and, based on such findings, and conceived to aspects of the invention described below.
[0015]
　(1)
　forming a blank from steel sheet,
　and performing a first hardening of the blank,
　after the first hardening, and performing a second hardening of the blank
　has a ,
　performs the first quenching process is
　a process of heating the blank to (Ac3 point -50) ° C. or higher 1200 ° C. or less of the first temperature to 2 ° C. / sec or more average heating rate,
　the blank and a step of cooling to said first second temperature of 250 ° C. or less from the temperature
　has,
　the second step of performing quenching,
　the said blank second from the temperature (Ac3 point -50 ) and heating at ° C. or higher 1200 ° C. or less of the third temperature to 2 ° C. / sec or more average heating rate,
　a step of cooling the blank to a fourth temperature of 250 ° C. or less from the third temperature ,
　has,
　the first quench or the first Quenching or manufacturing method of a hot stamping molded article and performing molding of the blank in both of these.
[0016]
　(2)
　between the step of cooling the heating to the first temperature to the second temperature, in (1) further comprising the step of holding more than one second to the first temperature method for producing a hot stamp molding according.
[0017]
　(3)
　the third temperature, (Ac3 point -50), characterized in that ° C. at 1000 ° C. or less than (1) or method of manufacturing a hot stamp molded article according to (2).
[0018]
　(4)
　hot stamping according to any one of and performing heating from the second temperature to the third temperature at 5 ° C. / sec or more average heating rate (1) - (3) method of manufacturing the body.
[0019]
　(5)
　and wherein a said a third between the step of cooling the heating to said fourth temperature to a temperature, the third step of holding at least 0.1 second or less than 300 seconds to a temperature of to (1) to a manufacturing method of a hot stamping molded article according to any one of (4).
[0020]
　(6)
　step of performing the second quenching, characterized by having a step of cooling the blank at an average cooling rate of 20 ° C. / sec from 700 ° C. to a fifth temperature Ms point -50 ° C. (1) to a manufacturing method of a hot stamping molded article according to any one of (5).
[0021]
　(7)
　area fraction of fresh martensite and tempered martensite: total 80% or more,
　prior austenite grain size: 20 [mu] m or less, and
　the average particle size of the carbide: 0.5 [mu] m or less,
　in that it has a steel structure represented hot stamping molded article characterized by.
[0022]
　(8)
　hot stamping molded article according to the C content is characterized in that 0.60% by mass or more 0.27 wt% (7).
[0023]
　(9)
　Vickers hardness is equal to or not less than 550 Hv (7) or hot stamping molded product according to (8).
Effect of the invention
[0024]
　According to the present invention, it is possible to obtain a hot stamp molded body can be suppressed low stress fracture with high strength.
DESCRIPTION OF THE INVENTION
[0025]
　Hereinafter, embodiments of the present invention will be described.
[0026]
　It will be described first steel structure of the hot stamping member according to an embodiment of the present invention. Hot stamping molded body according to the present embodiment, the area fraction of fresh martensite and tempered martensite: total 80% or more, prior austenite grain size: 20 [mu] m or less, an average particle size of the carbide: 0.5 [mu] m or less, in the table and a steel structure to be. Hot stamping body is a molded body obtained through a hot stamp.
[0027]
　(Area fraction of fresh martensite and tempered martensite: 80% or more in total)
　fresh martensite and tempered martensite which contributes to the improvement of strength. Fresh martensite and sufficient strength area fraction is less than 80% in total of tempered martensite, e.g. 1900MPa or more tensile strength can not be obtained. Therefore, the area fraction of fresh martensite and tempered martensite is 80% or more in total. Although the mechanical properties of the material is dependent on the volume fraction of the tissue or phases, if isotropic is the steel structure, the volume fraction is equivalent to the area fraction. The area fraction can be measured more easily than the volume fraction. Therefore, in the present application, using the area fraction.
[0028]
　(Prior austenite grain size (formerly γ grain size): 20 [mu] m or less)
　the old γ grain size is the average grain size of old γ grains. In the old γ particle size of 20μm greater, it can not be obtained sufficient fracture toughness, easy to low-stress destruction occurs. Therefore, the old γ grain size is 20μm or less. From the viewpoint of improving the fracture toughness and low stress fracture suppression, the old γ grain size is preferably not 15μm or less, more preferably 10μm or less.
[0029]
　(Average particle size of the carbide: 0.5 [mu] m or less)
　the average particle diameter of 0.5 [mu] m greater than the carbide, low stress cracking is likely to occur that starting from the coarse carbides. Therefore, the average particle size of the carbide is 0.5μm or less. From the viewpoint of suppression of the low stress fracture, the average particle size of the carbide is preferably 0.3μm or less. The carbides include iron-based carbide and carbonitrides such as cementite and ε carbides.
[0030]
　Typical steel structure, for example, include ferrite, pearlite, upper bainite, lower bainite, residual austenite, the fresh martensite or tempered martensite, or any combination thereof. Here, an example of how to measure the area fraction of these tissues or phases.
[0031]
　Ferrite, pearlite, upper bainite, the measurement of the area fraction of lower bainite and tempered martensite, a sample is taken as an observation plane section parallel to the parallel and the thickness direction to the rolling direction of a steel plate. Then, by polishing the observation surface, and nital etching, electrolytic emission at 5000 × magnification range of the thickness from the surface of the steel sheet when the t from the depth of t / 8 to a depth of 3t / 8 of the steel sheet scanning electron microscope observation at (field emission scanning electron microscope FE-SEM). By this method, it is possible to identify ferrite, pearlite, upper bainite, lower bainite and tempered martensite. Such observation performed on 10 fields, ferrite from the average value of 10 fields, pearlite, upper bainite, the area fraction of lower bainite and tempered martensite is obtained. As described later, upper bainite, lower bainite and tempered martensite can be distinguished from each other by the presence or absence and the extension direction of the iron-based carbides within lath-like grains.
[0032]
　Upper bainite is a set of lath-like grains, including carbides between lath. Lower bainite is a set of lath-like grains, diameter therein contains more iron-based carbides 5 nm. Iron-based carbide contained in the lower bainite has a single variant, iron-based carbides present in one of the crystal grains is substantially elongated in a single direction. The term "substantially single direction", angular difference means directions within 5 °. Tempered martensite is a set of lath-like grains, diameter therein contains more iron-based carbides 5 nm. However, unlike lower bainite, iron-base carbide contained in the tempered martensite has a plurality of variants, iron-based carbides present in one of the crystal grains are extended in multiple directions. Accordingly, tempering the martensite and lower bainite can be a direction that the iron-based carbide is extended it is determined depending on whether multiple or single.
[0033]
　In the measurement of the area fraction of retained austenite, a sample was taken from the steel sheet, a portion to a depth of t / 4 from the surface of the steel sheet was chemically polished, the depth from the parallel surface of the steel sheet in the rolling surface of the t / 4 measuring the X-ray diffraction intensity in a plane. For example, the area fraction Sγ of residual austenite is expressed by the following equation.
　S [gamma] = (I 200f + I 220f + I 311f ) / (I 200b + I 211b ) ×
100 (I 200f , I 220f , I 311f are respectively face-centered cubic (fcc) lattice phase (200), (220), (311 intensity of the diffraction peaks of), I 200b , I 211b has a body-centered cubic lattice, respectively (bcc) phase (200), indicating the intensity of the diffraction peak of (211).)
[0034]
　Fresh martensite and retained austenite, because they are not sufficiently corrosion by nital etching, can be distinguished ferrite, pearlite, upper bainite, the lower bainite and tempered martensite. Therefore, it is possible to identify the area fraction of fresh martensite by reducing the area fraction Sγ of residual austenite an area fraction of the remainder of the FE-SEM observation.
[0035]
　Ferrite is a grain bulk, does not include the infrastructure of the lath or the like therein. Perlite is a tissue ferrite and cementite becomes layered alternately. For example, ferrite layered in pearlite distinguish from ferrite of the bulk.
[0036]
　The particle size of the carbide refers to a circle-equivalent diameter determined from the area of ​​the measured the carbide in the observation plane of the specimen. Density and composition of carbides, for example, energy dispersive X-ray spectroscopy: transmission electron microscope equipped with analysis function by (energy dispersive X-ray spectrometry EDX) (transmission electron microscope: TEM) or three-dimensional atom probe electrolyte ion microscope can be measured using a (atom probe field ion microscope AP-FIM).
[0037]
　It will now be described chemical composition suitable steel sheet hot stamping molded article and a manufacturing according to an embodiment of the present invention. As mentioned above, hot stamping molded body according to the embodiment of the present invention is manufactured through at least two quenching blanking and blanking material of the steel sheet. Therefore, the chemical composition of the hot stamping material and steel sheet not only properties of a hot stamping molded article is obtained by considering these processes. In the following description, "%" is a unit of content of each element contained in the hot stamping member and the steel sheet, in particular means "% by mass" unless otherwise specified. Hot stamping molded body according to the present embodiment, C: 0.27% ~ 0.60%, Mn: 0.50% ~ 5.00%, Si: 2.00% or less, P: 0.030% or less , S: 0.0100% or less, acid soluble Al (sol.Al): 0.100% or less, N: 0.0100% or less, B: 0.0000% ~ 0.0050%, Cr: 0.00% ~ 0.50%, Mo: 0.00% ~ 0.50%, Ti: 0.000% ~ 0.100%, Nb: 0.000% ~ 0.100%, V: 0.000% ~ 0 .100%, Cu: 0.000% ~ 1.000%, Ni: 0.000% ~ 1.000%, O: 0.00% ~ 0.02%, W: 0.0% ~ 0.1 %, Ta: 0.0% ~ 0.1%, Sn: 0.00% ~ 0.05%, Sb: 0.00% ~ 0.05%, As: 0.00% ~ 0.05 , Mg: 0.00% ~ 0.05%, Ca: 0.00% ~ 0.05%, Y: 0.00% ~ 0.05%, Zr: 0.00% ~ 0.05%, La0 .00% to 0.05%, or Ce: 0.00% to 0.05%, and the balance has Fe and impurities, in a chemical composition represented. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0038]
　(C: 0.27% ~ 0.60%)
　C greatly contributes to the improvement of strength at low cost. The C content is less than 0.27%, unless expensive element is contained, sufficient strength, for example, difficult to obtain a strength of at least 1900 MPa. Therefore, C content is preferably 0.27% or more, more preferably 0.35% or more, more preferably 0.40% or more. On the other hand, C content is 0.60 percent, which may hydrogen embrittlement characteristics deteriorate greatly. Therefore, C content is preferably 0.60%.
[0039]
　(Mn: 0.50% ~
　5.00%) Mn is to increase the hardenability of the steel sheet by lowering the Ac3 point. The Mn content is less than 0.50%, sufficient hardenability can not be obtained. Therefore, Mn content is preferably 0.50% or more, and more preferably not less than 1.00%. On the other hand, the Mn content is 5.00% greater, sometimes the workability of the steel sheet before quenching degrades, preformed before quenching may become difficult. Further, a band-like structure due to segregation of Mn is likely to occur, the toughness of the steel sheet may be degraded. Therefore, Mn content is preferably 5.00% or less.
[0040]
　(Si: 2.00% or
　less) Si is contained as an impurity, for example, in the steel. The Si content is 2.00% greater, excessively high Ac3 point, or had to be carried out heating of the quenching at 1200 ° C. greater than the plating of the chemical conversion treatability and galvanized steel sheet lowered there is. Therefore, Si content is preferably not more than 2.00%, more preferably not more than 1.00%. Si is because of its effect of improving the hardenability of the steel sheet, Si may be contained.
[0041]
　(P: 0.030% or less)
　P is contained as an impurity, for example, in the steel. P is or deteriorate the workability of the steel sheet, or to degrade the toughness of the hot stamping molding. Therefore, P content is better as low as possible. In particular, a P content of 0.030 percent, a significant reduction in workability and toughness. Accordingly, P content is preferably 0.030% or less.
[0042]
　(S: 0.0100% or less)
　S is contained as an impurity, for example, in the steel. S is or deteriorate the formability of the steel sheet, or to degrade the toughness of the hot stamping molding. Therefore, S content is preferably as low as possible. In particular, S content is 0.0100% greater than the significant reduction in formability and toughness. Thus, S content is preferably not more 0.0100% or less, more preferably 0.0050% or less.
[0043]
　(Sol.Al:0.100%
　below) Sol. Al is contained as an impurity, for example, in the steel. sol. The Al content 0.100 percent, sometimes Ac3 point must excessively high, perform the heating of the quenching at 1200 ° C. greater. Therefore, sol. Al content is preferably 0.100% or less. sol. Because Al is having an effect of sound of the steel by deoxidation, sol. Al may be included.
[0044]
　(N: 0.0100% or less)
　N is contained as an impurity, for example, in the steel. N deteriorates the formability of the steel sheet. Therefore, N content is better as low as possible. In particular, N content is 0.0100% greater than the significant reduction in moldability. Therefore, N content is preferably 0.0100% or less.
[0045]
　B, Cr, Mo, Ti, Nb, V, Cu and Ni are good optional elements may optionally be contained in limits a predetermined amount to the hot stamping material, and the steel sheet.
[0046]
　(B: 0.0000% ~ 0.0050%)
　B is to improve the hardenability of the steel sheet. Thus, B may be contained. To obtain this effect sufficiently, B content is preferably 0.0001% or more. On the other hand, the B content is 0.0050% greater than the effect of the above effects are saturated, and the cost becomes disadvantageous. Therefore, B content is preferably 0.005% or less.
[0047]
　(Cr: 0.00% ~
　0.50%) Cr is to improve the hardenability of the steel sheet. Thus, Cr may be contained. To obtain this effect sufficiently, Cr content is preferably 0.18% or more. On the other hand, the Cr content is 0.50% greater, sometimes the workability of the steel sheet before quenching degrades, preformed before quenching may become difficult. Therefore, Cr content is preferably 0.50% or less.
[0048]
　(Mo: 0.00% ~
　0.50%) Mo is to improve the hardenability of the steel sheet. Therefore, Mo may be contained. To obtain this effect sufficiently, Mo content is preferably 0.03% or more. On the other hand, the Mo content of 0.50 percent, sometimes the workability of the steel sheet before quenching degrades, preformed before quenching may become difficult. Therefore, Mo content is preferably 0.50% or less.
[0049]
　(Ti: 0.000 Pasento ～ 0.100 Pasento, Nb: 0.000 Pasento ～ 0.100 Pasento, V: 0.000 Pasento ～ 0.100
　Pasento) Ti, Nb and V is a strengthening element, precipitation objects reinforced at dislocation strengthening through suppression of fine grain strengthening and recrystallization of a ferrite grain growth suppression, contribute to increase the strength of the steel sheet. To obtain this effect sufficiently, Ti content, Nb content and the V content are both preferably not less than 0.01%. On the other hand, Ti content, the Nb content and V content 0.100 percent, is that many will formability precipitation of carbonitrides degrades. Therefore, Ti content, Nb content and the V content are both preferably not more 0.100% or less.
[0050]
　(Cu: 0.000% ~ 1.000%, Ni: 0.000% ~
　1.000%) Cu and Ni contributes to improvement of strength. To obtain this effect sufficiently, Cu content and Ni content are both preferably not less than 0.01%. On the other hand, the Cu content or Ni content 1.000%, the pickling, weldability and hot workability may be deteriorated. Therefore, Cu content and Ni content are both preferably not more than 1.000%.
[0051]
　That, B: 0.0000% ~ 0.0050%, Cr: 0.00% ~ 0.50%, Mo: 0.00% ~ 0.50%, Ti: 0.000% ~ 0.100%, nb: 0.000% ~ 0.100%, V: 0.000% ~ 0.100%, Cu: 0.000% ~ 1.000%, or Ni: 0.000% ~ 1.000%, or it is preferred that any combination of these holds.
[0052]
　Hot stamping member and the steel plate, the following elements may be contained intentionally or inevitably limits a predetermined amount. That, O: 0.001% ~ 0.02%, W: 0.001% ~ 0.1%, Ta: 0.001% ~ 0.1%, Sn: 0.001% ~ 0.05%, sb: 0.001% ~ 0.05%, As: 0.001% ~ 0.05%, Mg: 0.0001% ~ 0.05%, Ca: 0.001% ~ 0.05%, Y: 0.001% ~ 0.05%, Zr: 0.001% ~ 0.05%, La0.001% ~ 0.05%, or Ce: 0.001% ~ 0.05%, or any of these the combination may be made up.
[0053]
　According to an embodiment of the present invention, it is possible to obtain a tensile strength of at least 1900 MPa, even if the low stress fracture occurs, the stress breakdown occurs can be more than 1800 MPa. Then, using this hot stamping molded article for automotive parts, while achieving superior collision safety, it is possible to reduce the weight of the vehicle body. For example, the auto parts tensile strength are used steel plate of about 500 MPa, if the tensile strength is replaced with part of 2500MPa about hot stamping member, collision safety is neck properties of thickness, and collision safety When sex is assumed to be proportional to the plate thickness and the steel sheet strength, it is possible to reduce the thickness by the tensile strength is 5 times 1/5. The plate thickness decrease leads to very great effect in improving the weight and fuel economy of automobiles.
[0054]
　Next, a method for manufacturing the hot stamping member according to an embodiment of the present invention. In the manufacturing method of hot stamping member according to an embodiment of the present invention, to form a blank from steel having the above chemical composition is subjected to at least two quenching the blank, one or both of the two quenching in performing the molding of the blank.
[0055]
　First quench (first heat treatment) is mainly carried out an average particle size of carbides in the hot stamping body in order to 0.5μm or less. Therefore, the first thermal treatment after the steel sheet steel structure, easy to contain fine carbides bainite, high percentage of fresh martensite and tempered martensite, that the proportion of includes easy ferrite and pearlite coarse carbides is low preferable. Specifically, it is preferably bainite, the total area fraction of fresh martensite and tempered martensite of 80% or more. Bainite, fresh martensite and tempered martensite is also referred to as a low temperature transformation structure, the steel structure comprising these 80% are very fine. If first thermal treatment after the steel structure fine, the steel structure after the second quenching (second thermal treatment) is also likely to be fine, easy low stress fracture is suppressed. The number density of carbides in the first steel sheet after heat treatment is preferably 0.50 pieces / [mu] m 2 is at least. This causes the finely dispersed carbides become nucleation sites inverse transformation to γ during the second thermal treatment heating, the second old γ grain size after heat treatment (formerly γ grain size in the hot stamping molded article) the in order to easily to 20μm or less. Further, in order to facilitate an average particle size of carbides in the hot stamping body 0.5μm or less, and preferably the average particle size of the carbide in the steel sheet after the first heat treatment small.
[0056]
　(Formation of the blank)
　steel sheet by blanking by the shear cutting or stamping or the like to form a blank. Vickers hardness of the steel sheet used in this embodiment is, for example, 500Hv or less, preferably 450Hv or less. If the Vickers hardness is less 500 Hv, it is possible to perform the blanking easily. Further, according to this embodiment, even the Vickers hardness of the steel sheet 500Hv or less, it is possible to obtain sufficient strength, for example a tensile strength of at least 1900 MPa.
[0057]
　(First hardening (first heat treatment))
　In the first
[0058]
　In the first temperature is lower than (Ac3 point -50 ° C.), carbides in the blank is not sufficiently dissolved, it is difficult to an average particle size of carbides in the hot stamping body 0.5μm or less. Accordingly, the first temperature is (Ac3 point -50 ° C.), is preferably 900 ° C. or higher, more preferably 1000 ° C. or higher. On the other hand, in the first temperature is 1200 ° C. greater, it is only the effect is saturated and the cost required for heating is increased. Accordingly, the first temperature is 1200 ° C. or less.
[0059]
　In a less than 1 in average heating rate 2 ° C. / sec to a temperature, coarsened old γ grains during warm, even if the second quenching to the old γ grain size of the hot stamping body 20μm or less it is difficult. Therefore, the average heating rate up to the first temperature is 2 ° C. / sec or more, preferably 5 ° C. / sec or more, more preferably 10 ° C. / sec or more, more preferably at 100 ° C. / sec or higher is there. Heating method is not particularly limited, for example, ambient heating, electric heating and infrared heating is exemplified.
[0060]
　Preferably hold time of 1 second or more to the first temperature. Held at the time is less than one second, sometimes carbides are not sufficiently dissolved. Therefore, the retention time is preferably not less than 1 second, more preferably not less than 100 seconds. On the other hand, the holding time is 600 seconds, greater than its effect is saturated, the productivity decreases, but only cost increases. Therefore, the retention time is preferably less 600 seconds.
[0061]
　A cooling stop temperature in the second temperature is 250 ° C. greater, easily formed comprises tends ferrite and pearlite coarse carbides, easy low-temperature transformation structure is difficult to produce that includes fine carbides. Thus, the second temperature is 250 ° C. or less.
[0062]
　During cooling from the first temperature to a second temperature, in the temperature range from 700 ° C. to 500 ° C., an average cooling rate is preferably 10 ° C. / sec or more. This is to avoid the ferrite transformation and pearlite transformation.
[0063]
　The temperature range from the first temperature to 700 ° C., cooling accompanying the transport of the blank may be performed. Cooling method is not particularly limited, for example, gas cooling and water cooling is exemplified. If gas cooling or water cooling is provided, it is preferable to apply tension to the blank as the blank material is not deformed due to thermal stress. In heat removal from the mold and pressed in the mold may be cooled blank. Water in the mold may be cooled blank by spraying the blank. If cooling in the mold, may be finished first heat treatment the blanks by pressing in the state of the plate in a flat mold, a mold in the shape of a hot stamping body during first heat treatment the blank may be pressed. In two steps of first thermal treatment and the second heat treatment may be processed into the shape of a hot stamping molded article.
[0064]
　Incidentally, Ac3 point (℃) can be calculated by the following equation. Here, [X] represents the content of the element X (mass%).
　Ac3 point 910-203√ =
　　　　　[C] -30 [Mn] -11 [Cr] Tasu44.7 [Si] +400 [Al] +700 [P] -15.2
　　　　　[Ni] -20 [Cu] +400 [Ti] +104 [V] +31.5 [Mo]
[0065]
　(Second quenching (second thermal treatment))
　In the second
[0066]
　In the third temperature is lower than (Ac3 point -50 ° C.), reverse transformation is insufficient to gamma, it is difficult to obtain sufficient tensile strength, for example a tensile strength of at least 1900 MPa. Therefore, the third temperature is (Ac3 point -50 ° C.) or higher, preferably (Ac3 point -20 ° C.), and more preferably not less than Ac3 point. On the other hand, in the third temperature is 1200 ° C. greater than the old γ grains are coarsened, it is difficult to the old γ grain size of the hot stamping body 20μm or less. Therefore, the third temperature is 1200 ° C. or less, preferably 1000 ° C. or less, more preferably 900 ° C. or less, more preferably 850 ° C. or less.
[0067]
　In a third less than the average heating rate 2 ° C. / sec to a temperature, coarsened old γ grains during temperature rise, it is difficult to the old γ grain size of the hot stamping body 20μm or less. Therefore, the average heating rate up to the third temperature is 2 ° C. / sec or more, preferably 5 ° C. / sec or more, more preferably 10 ° C. / sec or more, more preferably at 100 ° C. / sec or higher is there. Heating method is not particularly limited, for example, ambient heating, electric heating and infrared heating is exemplified. If first blank shape flat after heat treatment, electrical heating of the three is most preferred. Electrical heating is because the highest heating rate can be achieved. If molded into first heat treatment has been performed, infrared heating of the three is most preferred. Be uniformly heat the shaped blank in an electrically heated is difficult, because the infrared heating can be achieved a high heating rate than ambient heating.
[0068]
　Preferably the retention time of 0.1 second to 300 seconds to a third temperature. The retention time is less than 0.1 sec, the reverse transformation is insufficient to gamma, sufficient tensile strength, for example, it may be difficult to obtain a tensile strength of at least 1900 MPa. Therefore, the retention time is preferably at least 0.1 seconds. On the other hand, the holding time is 300 seconds or more, the old γ grains are coarsened, it may be difficult to make the old γ grain size of the hot stamping body 20μm or less. Therefore, the retention time is preferably 300 seconds or less, more preferably 30 seconds or less.
[0069]
　In a fourth temperature 250 ° C. greater is the cooling stop temperature, quenching is insufficient, martensite hot stamping body is insufficient. Therefore, the fourth temperature is 250 ° C. or less, preferably Ms point (° C.) -50 ° C. or less.
[0070]
　During cooling to a fourth temperature, the temperature range up to Ms point -50 ° C. from 700 ° C., an average cooling rate is preferably 20 ° C. / sec or more. The average cooling rate is less than 20 ° C. / sec in the temperature range from 700 ° C. to Ms point -50 ° C., ferrite transformation, it occurs pearlite transformation or bainite transformation, the area fraction of fresh martensite and tempered martensite Total in may be less than 80%. Therefore, the average cooling rate in the temperature range up to Ms point -50 ° C. from 700 ° C. is preferably 20 ° C. / sec or more.
[0071]
　Incidentally, Ms point (℃) can be calculated by the following equation. Here, [X] represents the content of the element X (mass%).
　Ms point 539-423 = [C]
　　　　-30.4 [Mn] -17.7 [Ni] -12.1 [Cr] -7.5 [Mo]
[0072]
　The third upper limit of the cooling rate from the temperature to the fourth temperature is not particularly limited, the cooling rate is also industrially using a special apparatus for cooling is usually 2000 ° C. / sec or less. The cooling rate is generally a simple water cooling and at 1000 ° C. / sec or less, a simple mold cooling is 500 ° C. / sec or less. The upper limit of the cooling rate in the cooling from the first temperature to a second temperature is the same.
[0073]
　Cooling the third from the temperature to the fourth temperature of the blank is carried out in the mold. In heat removal from the mold may be cooled blank, the water in the mold may be cooled blank by spraying the blank.
[0074]
　In this way, it is possible to produce a hot stamping molded body according to the embodiment of the present invention.
[0075]
　After retrieving the hot stamping body from the mold, the hot stamping member may be subjected to heating within 6 hours at a temperature of 50 ℃ ~ 650 ℃. If the temperature of this heating is 50 ° C. ~ 400 ° C., fine carbides are precipitated in the martensite in the heating, hydrogen embrittlement characteristics are improved. If the temperature of this heating is 400 ~ 650 ° C., the alloy carbides or intermetallic compounds or both are precipitated during heating, the strength is increased by a particle dispersion strengthening.
[0076]
　Although time is not particularly limited to the first quench starts second quenching from the end, depending on the composition of the blank, fine carbides in the blank is grown by prolonged holding at room temperature, the second there is a possibility that the average particle size of carbides after quenching increases. For this reason, the above-mentioned time, preferably within one month, more preferably less than one week, more preferably within one day.
[0077]
　The first quench or second quench or both may be repeated twice or more. As the number of times of quenching, the old γ grain size of the hot stamping molded body tends to be small. As described above, the old γ grain size is preferably not 15 [mu] m or less, more preferably where at 10 [mu] m or less, as the number of times of quenching, tends to give the old γ grain size of less than or equal to or less or 10 [mu] m 15 [mu] m.
[0078]
　Next, an example of a manufacturing method of a preferred steel for the production of hot stamping molding. Suitable steel for the production of hot stamping molded article was subjected hot rolled steel sheet not subjected to annealing, hot-rolled annealed steel sheet subjected to annealing hot-rolled steel sheet, cold rolling the hot-rolled steel sheet or hot-rolled annealed steel sheet cold-rolled steel sheet remains cold rolling, may be any of cold-rolled annealed steel sheet subjected to annealing cold-rolled steel sheet.
[0079]
　In this example, first, a steel having the above chemical composition, was melted by a conventional method, to obtain a slab by continuous casting. To obtain a steel ingot by casting the steel may be a steel ingot slabbing to obtain a slab. From the viewpoint of productivity, preferred is continuous casting.
[0080]
　Casting speed of continuous casting, in order to effectively suppress center segregation and V-shape segregation of Mn, preferably less than 2.0 m / min. Further, since keeping the cleanliness of the surface of the slab well, and in order to ensure the productivity, the casting speed is preferably between 1.2 m / min or more.
[0081]
　Then, subjected to hot rolling a slab or steel strip. In hot rolling, preferably, the slab heating temperature was 1100 ° C. or more for a solution of the inclusions, the finishing temperature is 850 ° C. or higher. Coiling temperature is preferably a processability point of view from 500 ° C. or higher, and 650 ° C. or less from the viewpoint of suppression of the reduction in yield due to scale formation.
[0082]
　Thereafter, the descaling process performed on hot-rolled steel sheet obtained by hot rolling a pickling or the like. The hot-rolled steel sheet after descaling process can be used for the production of hot stamping molding.
[0083]
　It may be subjected to hot-rolled sheet annealing to the hot rolled steel sheet after descaling. It is also possible to use hot-rolled annealed steel sheet obtained by hot-rolled sheet annealing in the manufacture of hot stamping molding.
[0084]
　It may be subjected to cold rolling to hot-rolled annealed steel sheets after hot-rolled sheet annealing. The cold-rolled steel sheet obtained by cold rolling can be used for the production of hot stamping molding. If hot-rolled annealed steel sheet is rigid, it is preferable to increase the workability annealed before cold rolling. Cold rolling may be performed by a conventional method. Rolling reduction in cold rolling, in order to ensure a good flatness, preferably 30% or more, in order to avoid that the load becomes excessive, preferably 80% or less.
[0085]
　It may be subjected to cold-rolled sheet annealing the cold-rolled steel sheet. Can be used cold-rolled annealed steel sheet obtained by cold-rolled sheet annealing in the manufacture of hot stamping molding.
[0086]
　The hot-rolled sheet annealing and cold-rolled sheet annealing, may be subjected to annealing after subjected to a treatment such as degreasing in a conventional manner as needed. From viewpoints of and productivity to homogenize the steel structure, the annealing is preferably performed in a continuous annealing line. When performing annealing in a continuous annealing line, preferably, soaking at 1000 seconds or less than 1 sec in a temperature range of less than Ac3 point (Ac3 point + 100 ° C.), followed by 250 ° C. or higher 550 ° C. in a temperature range below it is preferable to maintain 30 minutes or less than 1 minute.
[0087]
　Hot-rolled steel sheet, hot-rolled annealed steel sheet may be plated cold-rolled steel sheet or cold-rolled annealed steel sheet. When subjected to zinc-based plating as a plating, from the viewpoint of productivity, preferably for a hot-dip galvanized in a continuous galvanizing line. In that case, may be subjected to annealing prior to hot-dip galvanized in a continuous galvanizing line, it may be subjected to zinc-based plating without performing annealing with the soaking temperature to a low temperature. Performing alloying treatment after hot-dip galvanized may be the galvannealed steel sheet. The zinc-based plating may be applied by electroplating. Examples of galvanizing, hot-dip galvanized, galvannealed, electro-galvanized, hot-dip zinc - aluminum alloy plating, electrolytic nickel - zinc alloy plating and electric iron - zinc alloy plating are exemplified. Coating weight of plating is not particularly limited, and may be attached amount comparable to the conventional coated steel sheet. Zinc plating, can be applied to at least a portion of the surface of the steel material, generally, galvanized steel sheets applied to the whole of one or both surfaces of the steel sheet.
[0088]
　The above embodiments are all merely illustrate concrete examples of implementing the present invention, in which technical scope of the present invention should not be limitedly interpreted. That is, the present invention is its technical idea or without departing from the essential characteristics thereof, can be implemented in various forms.
Example
[0089]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0090]
　(First experiment)
　were subjected to hot rolling a slab having the chemical compositions shown in Table 1. In hot rolling, the slab heating temperature was 1250 ° C., a finishing temperature of 930 ° C., the coiling temperature was 650 ° C.. The cooling to finishing temperature coiling temperature from (930 ℃) (650 ℃) , and the average cooling rate 20 ° C. / sec. In this way, the thickness was obtained hot rolled steel sheets of 1.6mm or 3.2 mm. Then, it was subjected to de-scale processing of hot-rolled steel sheet. The remainder of the chemical compositions shown in Table 1 is Fe and impurities. Underlined in Table 1 indicates that the value is out of range of the present invention.
[0091]
[Table 1]

[0092]
　Thereafter, the hot rolled steel sheet having a thickness of 3.2 mm, as follows, to produce cold rolled steel sheets, aluminum-plated steel sheet, a galvanized steel sheet and galvannealed steel sheet. First, the thickness is carried out hot-rolled sheet annealing of 2 hours at 600 ° C. in the hot-rolled steel sheet of 3.2 mm, resulting thickness reduction ratio is performed rolled 50% cold is a cold-rolled steel sheet of 1.6mm It was. It was then carried out annealing of a portion of the cold-rolled steel sheet in a continuous melt annealing equipment or continuous hot-dip plating equipment. In this annealing, after holding for 120 seconds cold-rolled steel sheet to 800 ° C., and held for 200 seconds to 400 ° C.. After annealing, the cold-rolled steel sheet in a molten aluminum plating at 500 ° C. below the temperature, hot dip galvanized or galvannealed was performed. In this way, as a steel sheet for hot stamping, prepared hot-rolled steel sheet, cold rolled steel, aluminum-plated steel sheet, a galvanized steel sheet and galvannealed steel sheet.
[0093]
　Thereafter, a blank by blanking the hot stamping steel plate was subjected to first quench (first heat treatment) and a second quenching of the blank (second heat treatment). The first heat treatment conditions and the second thermal treatment conditions shown in Table 2 and Table 3. Incidentally, their atmosphere heating by the first heat treatment, the holding temperature to 700 ° C. is air-cooled, from 700 ° C. to the cooling stop temperature was cooled at an average cooling rate of 50 ° C. / sec in the mold a flat plate shape. In the second heat treatment, the heating rate their atmosphere heating if: 50 ° C. / sec, in the case of 50 ° C. / sec than was electrically heated. From the holding temperature to 700 ° C. is air-cooled, from 700 ° C. to the cooling stop temperature was cooled at an average cooling rate of 100 ° C. / sec while press-molded in a mold. Thus, to produce a variety of hot stamping molding. It underlined in Table 2 and in Table 3 indicates that the numerical value is outside the scope of the present invention.
[0094]
[Table 2]

[0095]
[table 3]

[0096]
　The second thermal treatment before the steel structure after first heat treatment, and was observed a second steel structure after heat treatment. The results are shown in Tables 4 and 5. Observation method steel structure are as described above. Moreover, it is taken specimens conforming to JIS Z 2201 from the hot stamping material, and measure the maximum tensile strength by a tensile test according to JIS Z 2241. Test No. It performed five tensile test for each, the test an average value of the five maximum tensile strength No. And the tensile strength. The results are also shown in Tables 4 and 5. It was the average value tensile strength, when the low stress fracture occurs, the manufacturing conditions are the same, because a large variation tends to occur breaking stress. There true strain epsilon a and true stress sigma a for, for samples that break occurs before the expression 2 below is satisfied determines that a low stress fracture occurs, the material fracture occurred after the expression 2 is satisfied it is determined that the low stress fracture did not occur. In Equation 2, [Delta] [epsilon] a is the 0.0002, .DELTA..sigma a is "true strain" epsilon a Tasu0.0002 "true stress σ in the case of a + 1 " and "true strain" epsilon a true stress σ in the case of " a and the difference between "(.DELTA..sigma a = sigma a + 1 - [sigma] a ).
　　IIA al / Mo al = p al　;;; (式2)
[0097]
[Table 4]

[0098]
[table 5]

[0099]
　Table 4 and Table 5, invention examples within the scope of the present invention (Test No.2 ~ No.5, No.8 ~ No.16, No.21 ~ No.22, No.24 ~ No .27, No.30 ~ No.31, No.36 ~ No.40, No.46 ~ No.50, No.56 ~ No.63, in the No.69 ~ No.70), low stress fracture occurs or not, stress destruction even as occurred occurs is equal to or greater than 1800MPa.
[0100]
　Test No. In 1, for holding temperature of the first quench is too low, insufficient old γ grain size of the hot stamping molded article, the average particle size of the carbide is excessive, sufficient tensile strength could not be obtained. Test No. In 6, because you did not first quenching, insufficient old γ grain size of the hot stamping molded article, the average particle size of the carbide is excessive, low stress fracture occurs, sufficient tensile strength can not be obtained . Test No. In 7, since too cooling stop temperature of the first quench high, insufficient old γ grain size of the hot stamping material, the average particle size of the carbide is excessive, low stress fracture occurs, a sufficient tensile strength obtained did not.
[0101]
　Test No. In 17, since the average heating rate in the first quench it is too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 18, for holding the temperature of the first quench was too low, insufficient old γ grain size of the hot stamping material, the average particle size of the carbide is excessive, low stress fracture occurs, sufficient tensile strength to give It is did not. Test No. In 19, since the average heating rate of the second quench it was too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 20, the cooling stop temperature in the second quench is too high, insufficient total area fraction of fresh martensite and tempered martensite, sufficient tensile strength could not be obtained.
[0102]
　Test No. In 23, for holding the temperature of the first quench is too low, insufficient old γ grain size of the hot stamping molded article, the average particle size of the carbide is excessive, sufficient tensile strength could not be obtained. Test No. In 28, for holding the temperature of the first quench was too low, insufficient old γ grain size of the hot stamping material, the average particle size of the carbide is excessive, low stress fracture occurs, sufficient tensile strength to give It is did not. Test No. In 29, because you did not first quenching, insufficient old γ grain size of the hot stamping molded article, the average particle size of the carbide is excessive, low stress fracture occurs, sufficient tensile strength can not be obtained . Test No. In 32, since the average heating rate in the first quench it is too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 33, the cooling stopping temperature of the first quench is too high, the average particle size of the carbide of the hot stamping body is excessive, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 34, since the average heating rate of the second quench it was too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 35, the cooling stop temperature in the second quench is too high, insufficient total area fraction of fresh martensite and tempered martensite, sufficient tensile strength could not be obtained.
[0103]
　Test No. In 41, since the average heating rate in the first quench it is too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 42, for holding the temperature of the first quench was too low, insufficient old γ grain size of the hot stamping material, the average particle size of the carbide is excessive, low stress fracture occurs, sufficient tensile strength to give It is did not. Test No. In 43, the cooling stopping temperature of the first quench is too high, the average particle size of the carbide of the hot stamping body is excessive, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 44, since the average heating rate of the second quench it was too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 45, the cooling stop temperature in the second quench is too high, insufficient total area fraction of fresh martensite and tempered martensite, sufficient tensile strength could not be obtained.
[0104]
　Test No. In 51, since the average heating rate in the first quench it is too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 52, for holding the temperature of the first quench was too low, insufficient old γ grain size of the hot stamping material, the average particle size of the carbide is excessive, low stress fracture occurs, sufficient tensile strength to give It is did not. Test No. In 53, the cooling stopping temperature of the first quench is too high, the average particle size of the carbide of the hot stamping body is excessive, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 54, since the average heating rate of the second quench it was too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 55, the cooling stop temperature in the second quench is too high, insufficient total area fraction of fresh martensite and tempered martensite, sufficient tensile strength could not be obtained.
[0105]
　Test No. In 64, since the average heating rate in the first quench it is too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 65, for holding the temperature of the first quench was too low, insufficient old γ grain size of the hot stamping material, the average particle size of the carbide is excessive, low stress fracture occurs, sufficient tensile strength to give It is did not. Test No. In 66, the cooling stopping temperature of the first quench is too high, the average particle size of the carbide of the hot stamping body is excessive, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 67, since the average heating rate of the second quench it was too low, insufficient old γ grain size of the hot stamping molded article, low stress fracture occurs, sufficient tensile strength could not be obtained. Test No. In 68, the cooling stop temperature in the second quench is too high, insufficient total area fraction of fresh martensite and tempered martensite, sufficient tensile strength could not be obtained.
[0106]
　(Second experiment)
　In a second experiment, the test in the first experiment No. 10, No. 31, No. 37, No. 47 and No. To form a blank in the same manner as 58, the first quenching (first heat treatment) of the blank, a second quenching (second thermal treatment) and the third quench (third heat treatment) was performed. First thermal treatment conditions, the second heat treatment conditions and third heat treatment conditions shown in Table 6. As shown in Table 6, in the third heat treatment, when the heating rate is less than 50 ° C. / sec their atmosphere heating, in the case of 50 ° C. / sec than was electrically heated. From the holding temperature to 700 ° C. is air-cooled, from 700 ° C. to the cooling stop temperature was cooled at an average cooling rate of 100 ° C. / sec while press-molded in a mold. Thus, to produce a variety of hot stamping molding.
[0107]
[Table 6]

[0108]
　And, to observe the steel structure after the heat treatment of the third. The results are shown in Table 7. Observation method steel structure are as described above. Further, a tensile test was carried out in the same manner as in the first experiment. The results are also shown in Table 7.
[0109]
[Table 7]
[0110]
As shown in Table 7, in any of the inventive example, the invention example is not performed third quench (Test No.10, No.31, No.37, No.47 or No.58) than the former γ grain size is small, superior mechanical properties are obtained.
Industrial Applicability
[0111]The present invention is, for example, can be used in industry in which a suitable hot stamping molded article for automotive parts.

The scope of the claims
[Requested item 1]　Forming a blank from steel sheet,
and performing a first hardening of the blank,
after the first hardening, and performing a second hardening of the blank,　it has a,
the second step for one quenching,
a process of heating the blank to (Ac3 point -50) ° C. or higher 1200 ° C. or less of the first temperature to 2 ° C. / sec or more average heating rate,
the said blank first a step of cooling from the first temperature to a second temperature of 250 ° C. or less,has a
step of performing the second quenching,
the said blank second temperature from (Ac3 point -50) of ° C. or higher and heating at an average heating rate of more than 2 ° C. / sec to a third temperature of 1200 ° C. or less,
a step of cooling the blank to a fourth temperature of 250 ° C. or less from the third temperature,
the organic and,
wherein the first quench or the second baked Method for producing a hot stamping body and performing been or molding of the blank in both of these.
[Requested item 2]
Between the step of cooling the heating to the first temperature to the second temperature, hot according to claim 1, characterized in that it comprises a step of holding more than one second to the first temperature method for producing a stamping member.
[Requested item 3]
It said third temperature is a manufacturing method of hot stamping molded article according to claim 1 or 2, characterized in that not more than 1000 ° C. (Ac3 point -50) ° C. or higher.
[Requested item 4]
Production of hot stamping molded article according to any one of claims 1 to 3, characterized in that in said second temperature a third average heating rate heating above 5 ° C. / sec to a temperature Method.
[Requested item 5]
Claims, characterized in that it has the the third between the step of cooling the heating to said fourth temperature to a temperature, the third step of holding at least 0.1 second or less than 300 seconds to a temperature of method for producing a hot stamp molding according to any one of 1 to 4.
[Requested item 6]
The step of performing the second quenching, claim 1, characterized in that it comprises a step of cooling the blank material at 20 ° C. / sec average cooling rate from 700 ° C. to a fifth temperature Ms point -50 ° C. method for producing a hot stamp molded article according to to any one of 5.
[Requested item 7]Area fraction of fresh martensite and tempered martensite: total 80% or more,
prior austenite grain size: 20 [mu] m or less, and
0.5μm or less, average particle size: carbide
and wherein a in steel structure represented hot stamping body.
[Requested item 8]
Hot stamping molded article according to claim 7, the C content is equal to or less than 0.60 mass% to 0.27 mass%.
[Requested item 9]
Hot stamping molded article according to claim 7 or 8 Vickers hardness is equal to or not less than 550 Hv.