The present invention relates to a steel sheet for hot stamping.
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 2014-161854 Patent Publication
Patent Document 2: Japanese Patent No. 5756773
Patent Document 3: Japanese Patent No. 5402191 discloses
Patent Document 4: Japanese Patent No. 5287770 discloses
Patent Document 5: JP 2014-118613 JP
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 suitable hot stamping steel in the manufacture of hot stamping molded article can be suppressed 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. The present inventors have found that, when this 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.
[0008]
　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.
[0009]
　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, bainite, fresh martensite and tempered martensite and the main phase, it is preferable to reduce the containing easily ferrite and pearlite coarse carbides, and, heating the hot stamp in order to obtain fine γ in, that you finely dispersing the carbide serving as the reverse transformation of nucleation sites in the steel sheet to γ ​​it it is preferable revealed. Furthermore, high number density of carbides, the proportion of carbides precipitated in addition the old γ grain boundaries among the carbides also revealed higher is preferable. The steel sheet having such a structure by hot stamping, excellent hot stamping body very fracture characteristics. The carbides, iron carbides, such as cementite and ε carbide, as well as carbides of TiC and alloying elements such as NbC. Carbonitrides are also included in the carbides.
[0010]
　Therefore, the present inventors have conducted further extensive studies. As a result, the present inventors found that by performing the cold-rolled sheet annealing under a predetermined condition, and found that a suitable steel is obtained for the production of hot stamping molded article having excellent fracture properties, like this based on the findings, and conceived to aspects of the invention described below.
[0011]
　(1)
　bainite area fraction of fresh martensite and tempered martensite: total 80% or more,
　the number density of carbides (pieces / [mu] m 2 product of)
　a steel sheet for hot stamping characterized by having a in steel structure represented.
[0012]
　(2)
　hot stamping steel sheet according to (1) the C content is not more than 0.60 mass% to 0.27 mass%.
[0013]
　(3)
　Vickers hardness is equal to or not less than 500 Hv (1) or (2) hot stamping steel sheet according to.
[0014]
　(4)
　Hot stamping steel sheet according to any one of and having a plating layer (1) to (3).
Effect of the invention
[0015]
　According to the present invention, it is possible to obtain a suitable hot stamping steel in the manufacture of hot stamping molded article can be suppressed low stress fracture with high strength.
DESCRIPTION OF THE INVENTION
[0016]
　Hereinafter, embodiments of the present invention will be described.
[0017]
　It will be described first steel structure of the hot stamping steel sheet according to the embodiment of the present invention. Hot stamping steel sheet according to the present embodiment, bainite area fraction of fresh martensite and tempered martensite: total 80% or more and a number density of carbides (pieces / [mu] m 2 ) and of the carbides in the old austenite grains the product of the percentage of precipitated carbides: has 0.50 or more, in the steel structure represented.
[0018]
　(Bainite area fraction of fresh martensite and tempered martensite: 80% or more in total)
　bainite, as the area fraction of fresh martensite and tempered martensite is high, a fine austenite during heating of the hot stamp (gamma) is easily obtained, the old γ grains of the hot stamping material becomes finer. Bainite, fresh martensite and tempered martensite is also referred to as a low temperature transformation structure. Bainite is less than 80% area fraction of fresh martensite and tempered martensite total, old γ grains of the hot stamping unit becomes coarse, sufficient fracture characteristics. Further, bainite, if the area fraction of fresh martensite and tempered martensite is 80% or more, the area fraction of inevitably pearlite is less than 20%, less likely to contain coarse carbides hot stamping member . Accordingly, bainite, area fraction of fresh martensite and tempered martensite is 80% or more in total, preferably 90% or more, more preferably 100%. Relatively, the area fraction of ferrite and pearlite is less than 20% in total, preferably not more than 10%, more preferably 0%. 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.
[0019]
　(Number density of carbides (pieces / [mu] m 2 ) the product of the proportion of carbides precipitated in the old gamma grains of carbide: 0.50 or higher)
　carbides becomes a nucleation site for the reverse transformation to gamma, carbide the more dense, fine γ is easily obtained during the heating of the hot stamping, old γ grains of the hot stamping material becomes finer. Since the old γ grain boundaries of the hot stamping steel sheet also becomes nucleation sites, carbides precipitated in old γ grain boundary is not substantially contribute to an increase in nucleation sites, carbides precipitated in old γ grains old γ grain boundaries It can be a different nucleation sites and. Then, the number density of carbides T (pieces / [mu] m 2 ), when the proportion of carbides precipitated in old γ grains of the carbide is M, in these product (T × M) is less than 0.50, hot old γ grains stamping unit becomes coarse, sufficient fracture characteristics. Therefore, the product (T × M) is 0.50 or more, preferably 0.60 or more, more preferably 0.70 or more. Not limit restriction of the product (T × M), it is difficult to produce a hot stamping steel plate to be greater than 10. Although the particle size of the carbide is not limited, it is preferred that the particle size is the number ratio of more coarse carbides 0.5μm is 0.15 or less. Carbides are classified as either of carbides precipitated in the carbide or the old γ grain boundaries deposited in a former γ grains.
[0020]
　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.
[0021]
　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.
[0022]
　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.
[0023]
　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).)
[0024]
　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.
[0025]
　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.
[0026]
　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).
[0027]
　Next, a description will be given chemical composition of the hot stamping steel sheet according to the embodiment of the present invention. As described below, hot stamping steel sheet according to the embodiment of the present invention, hot rolling, hot-rolled sheet annealing is manufactured by the cold rolling and cold-rolled sheet annealing or the like. Therefore, the chemical composition of the hot stamping steel sheet, not only the characteristics of the hot stamping steel plate, is taken into consideration these processes. In the following description, "%" is a unit of content of each element contained in the hot stamping steel sheet, in particular means "% by mass" unless otherwise specified. Hot stamping steel sheet 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.
[0028]
　(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, the C content of 0.60 percent, may be delayed fracture resistance of the hot stamping body is deteriorated. Moreover, not obtained sufficient processability, sometimes preforming before hot stamping becomes difficult. Therefore, C content is preferably not more than 0.60%, more preferably not more than 0.55%.
[0029]
　(Mn: 0.50% ~
　5.00%) Mn lowers the Ac3 point to improve the hardenability of the hot stamping steel plate. 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 hot stamping steel plate before quenching degrades, preformed before quenching may become difficult. Further, a band-like structure due to segregation of Mn becomes easily occurs, toughness of a hot stamping steel plate may be deteriorated. Therefore, Mn content is preferably 5.00% or less.
[0030]
　(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 chemical conversion treatability and galvanizing of the hot stamping steel plate is reduced sometimes or. 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 for hot stamping, Si may be contained.
[0031]
　(P: 0.030% or less)
　P is contained as an impurity, for example, in the steel. P is or deteriorate the workability of hot stamping steel plate, or 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.
[0032]
　(S: 0.0100% or less)
　S is contained as an impurity, for example, in the steel. S is or deteriorate the formability of hot stamping steel plate, or 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.
[0033]
　(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.
[0034]
　(N: 0.0100% or less)
　N is contained as an impurity, for example, in the steel. N deteriorates the formability of the hot stamping steel plate. 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.
[0035]
　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 steel plate.
[0036]
　(B: 0.0000% ~ 0.0050%)
　B is to improve the hardenability of the hot-stamping steel plate. 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.
[0037]
　(Cr: 0.00 The Pasento ～ 0.50
　Pasento) Cr is, to improve the hardenability of the hot stamp for steel plate. 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 hot stamping steel plate before quenching degrades, preformed before quenching may become difficult. Therefore, Cr content is preferably 0.50% or less.
[0038]
　(Mo: 0.00 The Pasento ～ 0.50
　Pasento) Mo is, to improve the hardenability of the hot stamp for steel plate. 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 hot stamping steel plate before quenching degrades, preformed before quenching may become difficult. Therefore, Mo content is preferably 0.50% or less.
[0039]
　(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 inhibition, contributes to an increase in the intensity of hot stamping steel plate. 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.
[0040]
　(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%.
[0041]
　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.
[0042]
　Hot stamping 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.
[0043]
　Vickers hardness of the hot stamping steel sheet according to the present embodiment is not limited, preferably not less than 500 Hv, and more preferably not less than 550 Hv.
[0044]
　According to an embodiment of the present invention, by performing the appropriate hot stamping, stresses in the hot stamping material, it is possible to obtain a tensile strength of at least 1900 MPa, even if the low stress fracture occurs, the fracture occurs it is possible to be more than 1800MPa. 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.
[0045]
　Next, a method for manufacturing the hot stamping steel sheet according to the embodiment of the present invention. In this manufacturing method, casting of steel having the above chemical composition, hot rolling of the slab, hot-rolled sheet annealing hot-rolled steel sheet, cold rolling the hot-rolled annealed steel sheet, the cold-rolled steel sheet cold-rolled sheet annealing, and cold performing a heat treatment or the like of the rolled annealed steel sheet.
[0046]
　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.
[0047]
　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.
[0048]
　Then, subjected to hot rolling a slab or steel strip. In hot rolling, the starting temperature from the viewpoint of more uniformly generate carbides and 1000 ° C. or higher 1300 ° C. or less. Finishing temperature of hot rolling is set to 850 ° C. or higher 1000 ° C. or less. The finishing temperature is lower than 850 ° C., the rolling load becomes excessive. The finishing temperature of 1000 ° C. greater than the old γ grain size is coarsened. Coiling temperature is set to 400 ° C. or higher 700 ° C. or less. The coiling temperature is lower than 400 ° C., the strength of the hot rolled steel sheet becomes excessive, breakage and defective shape is likely to occur during cold rolling. The coiling temperature 700 ° C. greater than the oxide on the surface of the hot-rolled steel sheet is excessively generated, pickling performance deteriorates.
[0049]
　Thereafter, the descaling process performed on hot-rolled steel sheet obtained by hot rolling a pickling or the like. Subjected to a hot-rolled sheet annealing the hot-rolled steel sheet after descaling treatment. Subjected to cold rolling to hot-rolled annealed steel sheets after hot-rolled sheet annealing. 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.
[0050]
　Then, subjected to cold-rolled sheet annealing cold-rolled steel sheet obtained by cold rolling. The cold-rolled sheet annealing, and heated to a first temperature of 1100 ° C. or less than Ac3 point, 1000 seconds or less than 1 second to a first temperature (heating time) and held, -150 ° C. below the second temperature until cool.
[0051]
　In the first temperature is lower than Ac3 point, carbide coarse carbides remained without sufficiently dissolved, insufficient number density of carbides after cooling. Accordingly, the first temperature is at least Ac3 point. In the first temperature is 1100 ° C. greater than the carbide saturation effect to dissolve, unnecessarily cost increases greatly. Further, in the first temperature is 1100 ° C. greater, gamma grains easily coarsen, old gamma particle also prone to those coarse hot stamping molding. Accordingly, the first temperature is preferably 1100 ° C. or less. Heating rate to a first temperature is not limited, for example, a 1 ° C. / sec ~ 5000 ° C. / sec. As the heating method, preferably easily obtained electric heating is heating rate of more than 100 ° C. / sec.
[0052]
　The heating time is less than 1.0 seconds, carbide coarse carbides remained without sufficiently dissolved, insufficient number density of carbides after cooling. Therefore, the heating time is more than 1.0 seconds. The heating time is 1000 seconds, more than carbides saturated the effect of dissolving, unnecessarily cost increases greatly. Further, the heating time is 1000 seconds greater, gamma grains easily coarsen, old gamma particle also prone to those coarse hot stamping molding. Therefore, the heating time is preferably 1000 seconds or less.
[0053]
　In the second temperature is -150 ° C. greater is the cooling stop temperature, it may include the residual γ in cold-rolled annealed steel sheet. The inclusion of residual gamma cold-rolled annealed steel sheets, the concentration of dissolved C in the residual gamma, hardly precipitated carbides enough during the heat treatment after the cold rolled sheet annealing. Thus, the second temperature is -150 ° C. or less. During cooling from the first temperature to a second temperature, average cooling rate in the temperature range from the first temperature to 100 ° C. (first temperature range) is set to 1000 ° C. / sec or more, -150 from 100 ° C. the average cooling rate in the temperature range up to ° C. is a 50 ° C. / sec or higher. The average lower than the cooling rate is 1000 ° C. / sec in the temperature range from the first temperature to 100 ° C., carbide tends to precipitate in the old γ grain boundaries after martensitic transformation. The average cooling rate is less than 50 ° C. / sec in the temperature range from 100 ° C. to -150 ° C., tends to solute C is concentrated γ during cooling, residual γ is likely to remain.
[0054]
　Cold-rolled sheet annealing may be performed two or more times. As performing cold-rolled sheet annealing, the old γ grains become finer, leading to comminution of old γ grains of the hot stamping molding.
[0055]
　After the cold rolled sheet annealing is carried out heat treatment of cold-rolled annealed steel sheet (reheated). In this heat treatment, heated to 100 ° C. or higher 300 ° C. or less of the temperature (heating temperature), this temperature 10 minutes or more 480 minutes or less (heating time) and held, cooled to about room temperature. By this heat treatment, it is possible to finely dispersed carbide former γ intragranular.
[0056]
　The heating temperature is lower than 100 ° C., carbides are not sufficiently precipitate. Therefore, the heating temperature is 100 ° C. or higher. The heating temperature is 300 ° C. greater than the carbide grows coarsely number density of the carbides is reduced. Therefore, the heating temperature is 300 ° C. or less. In less than 10 minutes heating time, carbide is not sufficiently precipitated. Therefore, the heating time is 10 minutes or more. The heating time is 480 minutes excess, carbides grow coarsely number density of the carbides is reduced. Therefore, the heating time is less than 480 minutes.
[0057]
　In this way, it is possible to produce a hot stamping steel plate.
[0058]
　It may be subjected to a plating on the hot stamp for steel plate. 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.
[0059]
　Next, an example of a manufacturing method of a hot stamping molded article using a hot stamping steel sheet according to the embodiment of the present invention. In this example, the blank from the hot stamping steel sheet according to the embodiment form of the present invention, subjected to quenching the blank, performs shaping of the blank during the hardening.
[0060]
　(Formation of the blank)
　shear cutting hot stamping steel sheet, and blanking to form a blank by laser cutting or stamping or the like. Vickers hardness of the hot stamping steel sheet according to the present embodiment is, for example, 500Hv or more. If the Vickers hardness is high, it is preferable to perform laser cutting.
[0061]
　(Quenching)
　in hardening, the blank is heated at an average heating rate of more than 2 ° C. / sec to a third temperature of 1000 ° C. or less than Ac3 point, blank the third temperature 3 minutes or 0.1 seconds retention and, cooling the blank from the third temperature to a fourth temperature of 400 ° C. or less. Performed molded into this cooling, the temperature range up to 400 ° C. from Ar3 point, the average cooling rate to 100 ° C. / sec or more. By holding the blank in a third temperature, the steel structure at the start of the molding becomes γ single-phase structure, the main phase of the steel structure becomes martensite during cooling until further fourth temperature.
[0062]
　In a less than 3 in temperature Ac3 point, ferrite is included in the steel structure during molding, ferrite is grown during cooling, the area fraction of martensite may not gain sufficient strength is lower . In the third temperature is 1000 ° C. greater, with its effect is saturated, grains grow excessively gamma, become old gamma particle of hot stamping body is coarse, low stress cracking is likely to occur.
[0063]
　In a third less than the average heating rate 2 ° C. / sec to a temperature, gamma grains during heated becomes coarse, low stress fracture is likely to occur in hot stamping molding. The heating method is not limited, furnace heating, infrared heating, electric heating and the like. Electrical heating of these are most preferred. Electrical heating is because the highest average heating rate can be achieved. The higher the average heating rate, the γ grains easily miniaturized, high productivity is obtained.
[0064]
　In a third holding time is less than 0.1 seconds at a temperature of, insufficient reverse transformation to gamma, sufficient tensile strength, for example, it may be difficult to obtain a tensile strength of at least 1900 MPa. On the other hand, the retention time 3 minutes or more, grains are coarsened gamma, low stress fracture is likely to occur in hot stamping molding.
[0065]
　In a fourth temperature 400 ° C. greater than the quenching is insufficient, martensite hot stamping body is insufficient. During cooling to a fourth temperature, the temperature range up to 400 ° C. from Ar3 point, the average cooling rate to 100 ° C. / sec or more. The average cooling rate is less than 100 ° C. / sec at this temperature range, ferrite transformation, it occurs pearlite transformation or bainite transformation, the main phase is not the steel martensite is obtained, that sufficient strength can not be obtained is there. The Ar3 point or more, since the phase transformation of the ferrite transformation or the like does not occur, the average cooling rate is not limited. For example, even in a temperature range of not lower than Ar3 point, the average cooling rate may be 100 ° C. / sec or more.
[0066]
　The upper limit of the cooling rate from the third temperature to a fourth temperature is not 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.
[0067]
　The temperature range from the third temperature to 700 ° C., cooling accompanying the transport of the blank may be performed. 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.
[0068]
　The cooling rate in the temperature range of 400 ° C. or less is not limited. If the average cooling rate in the temperature range of 400 ° C. or less is less than 100 ° C. / sec, internal fine carbides tempered martensite or bainite obtained precipitated, residual of about several% gamma is obtained. These contribute to improvement of the ductility. To the average cooling rate be less than 100 ° C. / sec, for example, a steel sheet where or by press with a mold heated to a temperature between 400 ° C. from room temperature is a temperature of 400 ° C. from room removed from the press machine, or decrease the intentional cooling rate. If the average cooling rate in the temperature range of 400 ° C. or less is at 100 ° C. / sec or more, single martensite structure is obtained, generation of carbides in martensite is suppressed, particularly high strength can be obtained.
[0069]
　In this way, it is possible to produce a hot stamping molded article.
[0070]
　Incidentally, Ac3 point (℃) and Ar3 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] Tasu31.5
　[Mo] Ar @ 3 point
　　　　　901-325 = [C] +33 [Si] -92 ([Mn] + [Ni] / 2 + [Cr] / 2 + [Cu] / 2 + [Mo] / 2 )
[0071]
　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 in martensite is precipitated during heating, delayed fracture resistance and mechanical properties 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.
[0072]
　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
[0073]
　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.
[0074]
　(First experiment)
　was obtained hot rolled steel sheet by performing hot rolling of a slab having a chemical composition shown in Table 1. The remainder of the chemical compositions shown in Table 1 is Fe and impurities. In hot rolling, 1200 ° C. The starting temperature, the finishing temperature 900 ° C., the coiling temperature was 600 ° C.. In cooling from the finishing temperature to the coiling temperature, and the average cooling rate 20 ° C. / sec. Then subjected to descaling by pickling hot-rolled steel sheet, rolling reduction performs rolling of 60% cold, thick was obtained cold-rolled steel sheet of 1.6 mm. It was then carried out cold-rolled sheet annealing of the cold-rolled steel sheet in an electric heating equipment. The condition of cold-rolled sheet annealing are shown in Table 2. Table first temperature range in 2 is the temperature range from the heating temperature to 100 ° C., the second temperature range is a temperature range from 100 ° C. to the cooling stop temperature. The cold-rolled sheet annealing, and the average heating rate up to the heating temperature was 500 ° C. / sec. After the cold-rolled sheet annealing was carried out heat treatment of cold-rolled annealed steel sheet (reheated). Conditions for this heat treatment are also shown in Table 2.
[0075]
　After the heat treatment, molten cold-rolled annealed steel sheet aluminum plating was carried out galvanizing or galvannealing. When manufacturing a galvannealed steel sheet performs alloying process 550 ° C. After holding, cooled to room temperature, and wound. 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.
[0076]
　Then, a blank material and blanking the hot stamp for steel plate, it was hardening of the blank. Quenching was performed in one of the following two conditions A and conditions B. In condition A, and heated to 900 ° C. at an average heating rate of 10 ° C. / sec by atmosphere heating, and held for 2 minutes at 900 ° C., and air-cooled to 700 ° C., an average cooling rate of 100 ° C. / sec from 700 ° C. to 100 ° C. in the mold cooling. In condition B, rapidly heated to 900 ° C. at an average heating rate of 100 ° C. / s by electrical heating, and held for 1 second at 900 ° C., and air-cooled to 700 ° C., the average cooling 100 ° C. / s from 700 ° C. to 100 ° C. was mold cooled at a rate. Thus, to produce a variety of hot stamping molding. Table 1 Underline in to Table 3 indicates that the numerical value is outside the scope of the present invention.

WE claims

Bainite area fraction of fresh martensite and tempered martensite: total 80% or more,
　the number density of carbides (pieces / [mu] m 2 ) the product of the proportion of carbides precipitated in the prior austenite grains of carbides: 0. 50 or more,
　a steel sheet for hot stamping characterized by having a in steel structure represented.
[Requested item 2]
　Hot stamping steel sheet according to claim 1, the C content is equal to or less than 0.60 mass% to 0.27 mass%.
[Requested item 3]
　Hot stamping steel sheet according to claim 1 or 2 Vickers hardness is equal to or not less than 500 Hv.
[Requested item 4]
　Hot stamping steel sheet according to any one of claims 1 to 3, wherein a plated layer.