Invention name: Hot stamp molded article
Technical field
[0001]
　The present invention relates to a hot stamp molded body having excellent shock absorbing ability, which is used for structural members and reinforcing members of automobiles and structures that require strength.
Background technology
[0002]
　In recent years, there has been a demand for weight reduction of automobile bodies from the viewpoint of environmental protection and resource saving, and therefore, the application of high-strength steel sheets to automobile members is accelerating. However, since the formability deteriorates as the strength of the steel sheet increases, the formability of a high-strength steel sheet into a member having a complicated shape becomes an issue.
[0003]
　In order to solve such a problem, the application of hot stamping in which a steel sheet is heated to a high temperature in the austenite region and then press-formed is being promoted. Hot stamping is attracting attention as a technology that achieves both molding into automobile members and ensuring strength because quenching is performed in the mold at the same time as press working.
[0004]
　On the other hand, a molded product obtained by hot stamping a high-strength steel sheet is required to have the ability to absorb impact at the time of collision.
[0005]
　As a technique for meeting this demand, Patent Document 1 states that by annealing a steel plate for hot stamping and concentrating Mn and Cr in the carbide to make a carbide difficult to dissolve, austenite is produced by these carbides during hot stamping. A technique for suppressing the growth of manganese and making it finer is disclosed.
[0006]
　Patent Document 2 discloses a technique for finely granulating austenite by raising the temperature at a heating rate of 90 ° C./s or less during hot stamp heating.
[0007]
　Patent Document 3, Patent Document 4, and Patent Document 5 also disclose a technique for finely granulating austenite to improve toughness.
Prior art literature
Patent documents
[0008]
Patent Document 1: International Publication No. 2015/147216
Patent Document 2: Patent No. 5369714
Patent Document 3: Patent Document 5114691
Patent Document 4: Japanese Patent Application Laid-Open No. 2014-15638
Patent Document 5: Japanese Patent Application Laid-Open No. 2002-309345 Gazette
Outline of the invention
Problems to be solved by the invention
[0009]
　However, with the techniques disclosed in Patent Documents 1 to 5, it is difficult to obtain finer-grained austenite, and it is not expected to obtain a shock absorbing ability higher than that of the conventional one.
[0010]
　In view of the problems of the prior art, it is an object of the present invention to secure a better shock absorbing ability in a hot stamped body of a high-strength steel plate, and an object of the present invention is to provide a hot stamped body that solves the problem. And.
Means to solve problems
[0011]
　The present inventors have diligently studied a method for solving the above problems. As a result, the average crystal grain size of the former austenite was set to 3 μm or less, and one or two of Nb and Mo were dissolved in the former austenite grain boundaries to increase the embrittlement strength of the grain boundaries, as compared with the conventional case. It has been found that excellent shock absorption capacity can be obtained.
[0012]
　The invention of the present application has been further studied based on the above findings, and the gist thereof is as follows.
[0013]
　(1) Ingredient composition is C: 0.15% or more, less than 0.35%, Si: 0.005% or more, 0.25% or less, Mn: 0.5% or more, 3.0 in mass%. % Or less, sol. Al: 0.0002% or more, 3.0% or less, Cr: 0.05% or more, 1.00% or less, B: 0.0005% or more, 0.010% or less, Nb: 0.01% or more, 0.15% or less, Mo: 0.005% or more, 1.00% or less, Ti: 0% or more, 0.15% or less, Ni: 0% or more, 3.00% or less, P: 0.10% Hereinafter, S: 0.10% or less and N: 0.010% or less are contained, the balance is Fe and unavoidable impurities, and the microstructure contains bainite having an average crystal grain size of 3 μm or less, and further. , Lower bainite, martensite and tempered martensite in an area ratio of 90% or more, Z = (mass% of one or two of Nb and Mo at grain boundaries) / (Nb at dissolution and A hot stamped molded product having a defined grain boundary solid solution ratio Z of 0.3 or more (mass% of one or two types of Mo).
[0014]
　(2) The hot stamp molded product according to (1) above, which has a plating layer.
The invention's effect
[0015]
　According to the present invention, it is possible to provide a hot stamp molded product having high strength and superior shock absorbing ability as compared with the conventional one.
A brief description of the drawing
[0016]
[Fig. 1] Fig. 1 is a diagram showing a shape of a test piece when measuring a grain boundary solid solution ratio.
Mode for carrying out the invention
[0017]
　A feature of the present invention is that the average crystal grain size of the former austenite is 3 μm or less, and one or two of Nb and Mo are dissolved in the former austenite grain boundaries to increase the embrittlement strength of the grain boundaries. .. As a result of diligent studies, the present inventors have found that the above-mentioned structure can be obtained by the following method.
[0018]
　As a first step, the amount of molten steel cast per unit time is controlled. As a result, the microsegregation of Mn in the steel piece is suppressed, the precipitation of Mo and Nb is suppressed, and the solid solution amount of Mo and Nb in the steel is increased.
[0019]
　When the amount of molten steel cast per unit time is controlled to reduce the microsegregation of Mn, the trap site of P disappears, so that P segregates into the old austenite grain boundaries during finish rolling. Then, although the old austenite grain boundaries are finely divided, the embrittlement strength of the grain boundaries is lowered, and the shock absorbing ability cannot be sufficiently obtained. This is because the affinity between Mn and P is high, so that the segregation of Mn functions as a trap site for P, and by eliminating the segregation, P diffuses into the old austenite grain boundaries. In the present invention, this problem is solved by controlling the rolling conditions in the second stage.
[0020]
　As the second step, by controlling the rolling reduction, temperature, cooling conditions after rolling, and winding temperature of hot finish rolling, Mn concentration in carbide is suppressed and easily soluble fine carbide is generated. In addition, high density dislocations are introduced into the steel. In the present invention, both finely dispersed carbides and high-density dislocations become reverse transformation sites of austenite to refine the former austenite grains. In order to effectively function as a reverse transformation site, it is desirable that the carbide is easily dissolved. Therefore, it is important not to concentrate elements such as Mn and Cr that inhibit the dissolution of carbides in the carbides.
[0021]
　Further, by suppressing the precipitation of Mo and Nb and dissolving Nb and Mo in the grain boundaries of the former austenite, the segregation site of P is occupied by Nb and Mo, and the segregation of P to the former austenite is eliminated. As a result, not only the improvement of the grain boundary strength due to Mo or Nb but also the reduction of the embrittlement strength of the grain boundary can be suppressed.
[0022]
　As a third step, by controlling the rate of temperature rise during hot stamp heating, both easily soluble microcarbides and high-density dislocations become nucleation sites for the former austenite. Thereby, the average crystal grain size of the old austenite in the hot stamp molded product can be controlled to 3 μm or less.
[0023]
　Further, the precipitation of NbC and MoC during heating is suppressed, and the solid solution ratio of one or two types of Nb and Mo at the grain boundaries of the former austenite is increased. In order to suppress the precipitation of Mo and Nb, it is necessary to set the rate of temperature rise during hot stamp heating to at least 100 ° C./s or more.
[0024]
　The shock absorption capacity can be evaluated by the brittle fracture surface ratio of the Charpy impact test. The difference in brittle fracture surface ratio is due to the difference in grain boundary strength. The grain boundary strength is determined by the microstructure and type of the molded product (martensite, tempered martensite, lower bainite, etc.), the average grain size of the former austenite, and the concentration of grain boundary solid solution elements such as Nb and Mo.
[0025]
　The grain boundary strength can be increased by increasing the grain boundary solid solution amount of Nb and Mo. However, since Nb and Mo easily combine with C in steel to form carbides at a temperature of 500 ° C. or higher. It is necessary to consistently control the manufacturing process from continuous casting, hot rolling, and hot pressing to suppress the precipitation of these elements. That is, in order to increase the grain boundary solid solution amount of Nb and Mo, it is necessary to satisfy the conditions described later in all the steps from the first step to the third step described above.
[0026]
　Hereinafter, the hot stamp molded product of the present invention and a method for producing the same will be described in detail.
[0027]
　First, the reason for limiting the component composition of the hot stamp molded product according to the present invention will be described. Hereinafter,% related to the component composition means mass%.
[0028]
　"C: 0.15% or more and less than 0.35%"
　C is an important element for obtaining a tensile strength of 1500 MPa or more. If it is less than 0.15%, martensite is soft and it is difficult to secure a tensile strength of 1500 MPa or more, so C is set to 0.15% or more. It is preferably 0.20% or more. On the other hand, it is set to less than 0.35% in consideration of the required balance between shock absorption capacity and strength. It is preferably less than 0.34%.
[0029]
　"Si: 0.005% or more, 0.25% or less"
　Si is an element that enhances the deformability and contributes to the improvement of the shock absorption capacity. If it is less than 0.005%, the deformability is poor and the shock absorbing ability deteriorates. Therefore, 0.005% or more is added. It is preferably 0.01% or more. On the other hand, if it exceeds 0.25%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the average crystal grain size of the old austenite cannot be controlled to 3 μm. Therefore, the upper limit is set to 0.25%. It is preferably 0.22% or less.
[0030]
　"Mn: 0.5% or more, 3.0% or less"
　Mn is an element that contributes to the improvement of strength by strengthening solid solution. If it is less than 0.5%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to secure a tensile strength of 1500 MPa or more. Therefore, 0.5% or more is added. It is preferably 0.7% or more. On the other hand, if it is added in excess of 3.0%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the average crystal grain size of the old austenite cannot be controlled to 3 μm or less. And. It is preferably 2.5% or less.
[0031]
　"Sol.
　Al : 0.0002% or more, 3.0% or less" Al is an element that deoxidizes molten steel and makes the steel sound. If it is less than 0.0002%, deoxidation is sufficient and coarse oxides are formed, causing premature fracture. Al is 0.0002% or more. It is preferably 0.0010% or more. On the other hand, if it is added in excess of 3.0%, coarse oxides are formed and the toughness is impaired, so the content should be 3.0% or less. It is preferably 2.5% or less, more preferably 0.5% or less.
[0032]
　"Cr: 0.05% or more, 1.00% or less"
　Cr is an element that contributes to the improvement of strength by strengthening solid solution. If it is less than 0.05%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to secure a tensile strength of 1500 MPa or more. Therefore, 0.05% or more is added. It is preferably 0.1% or more. On the other hand, if it is added in excess of 1.00%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the particle size of the old austenite cannot be controlled to 3 μm or less. Therefore, the upper limit is 1.00%. .. Preferably, it is 0.8% or less.
[0033]
　"B: 0.0005% or more, 0.010% or less"
　B is an element that contributes to the improvement of strength by strengthening the solid solution. If it is less than 0.0005%, the solid solution strengthening ability is poor and martensite becomes soft, and it is difficult to secure a tensile strength of 1500 MPa or more. Therefore, 0.0005% or more is added. It is preferably 0.0008% or more. On the other hand, if it is added in excess of 0.010%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the average crystal grain size of the old austenite cannot be controlled to 3 μm or less. Therefore, the upper limit is 0.010%. And. Preferably, it is 0.007% or less.
[0034]
　"Nb: 0.01% or more, 0.15% or less"
　Nb is an element that dissolves in the grain boundaries of old austenite and increases the strength of the grain boundaries. Further, since Nb inhibits the segregation of P at the grain boundary by being dissolved in the grain boundary, the embrittlement strength of the grain boundary is improved. Therefore, 0.01% or more is added. It is preferably 0.030% or more. On the other hand, if it is added in excess of 0.15%, it tends to precipitate as carbide and the amount of solid solution at the grain boundary decreases, so the content is 0.15% or less. It is preferably 0.12% or less.
[0035]
　"Mo: 0.005% or more, 1.00% or less"
　Mo is an element that dissolves in the grain boundaries of former austenite and increases the strength of the grain boundaries. Further, since Mo inhibits the segregation of P at the grain boundary by being dissolved in the grain boundary, the embrittlement strength of the grain boundary is improved. Therefore, 0.005% or more is added. It is preferably 0.030% or more. On the other hand, if it is added in excess of 1.00%, it tends to precipitate as carbide and the amount of solid solution at the grain boundary decreases, so the content is set to 1.00% or less. It is preferably 0.80% or less.
[0036]
　"Ti: 0% or more, 0.15% or less"
　Ti is not an essential element, but it may be added as necessary because it is an element that contributes to the improvement of strength by strengthening the solid solution. When Ti is added, it is preferably 0.01% or more in order to obtain the effect of the addition. It is preferably 0.02%. On the other hand, if it is added in an amount of more than 0.15%, coarse carbides and nitrides are formed and premature fracture is caused. It is preferably 0.12% or less.
[0037]
　"Ni: 0% or more, 3.00% or less"
　Ni is not an essential element, but it may be added if necessary because it is an element that contributes to the improvement of strength by strengthening the solid solution. When Ni is added, it is preferably 0.01% or more in order to obtain the effect of the addition. It is preferably 0.02%. On the other hand, if it is added in excess of 3.00%, the steel becomes brittle and causes early fracture, so the content should be 3.00% or less. It is preferably 2.00% or less.
[0038]
　"P: 0.10% or less"
　P is an impurity element, which easily segregates at grain boundaries and reduces the embrittlement strength of grain boundaries. If it exceeds 0.10%, the embrittlement strength of the grain boundaries is remarkably lowered and early fracture is caused. Therefore, P is set to 0.10% or less. It is preferably 0.050% or less. The lower limit is not particularly limited, but if it is reduced to less than 0.0001%, the de-P cost will increase significantly and it will be economically disadvantageous. Therefore, 0.0001% is a practical lower limit on the practical steel sheet.
[0039]
　"S: 0.10% or less"
　S is an impurity element and is an element that forms inclusions. If it exceeds 0.10%, inclusions are formed and cause premature fracture, so S is set to 0.10% or less. It is preferably 0.0050% or less. The lower limit is not particularly limited, but if it is reduced to less than 0.0015%, the cost of removing S is significantly increased, which is economically disadvantageous. Therefore, 0.0015% is a practical lower limit on the practical steel sheet.
[0040]
　"N: 0.010% or less"
　N is an impurity element and forms a nitride to cause premature fracture. Therefore, it is set to 0.010% or less. It is preferably 0.0075% or less. The lower limit is not particularly limited, but if it is reduced to less than 0.0001%, the N removal cost will increase significantly and it will be economically disadvantageous. Therefore, 0.0001% is a practical lower limit on the practical steel sheet.
[0041]
　The rest of the component composition is Fe and impurities. Examples of impurities include elements that are unavoidably mixed from steel raw materials or scrap and / or in the steelmaking process and are allowed as long as they do not impair the characteristics of the hot stamped article of the present invention.
[0042]
　Next, the reason for limiting the microstructure of the hot stamp molded product of the present invention will be described.
[0043]
　"The average crystal grain size of old austenite is 3.0 μm or less"
[0044]
　The average crystal grain size of the former austenite is an important tissue factor for ensuring excellent strength and the effect of suppressing premature fracture. According to the studies by the present inventors, in order to obtain the shock absorbing ability required for the hot stamped molded product, the smaller the particle size of the old austenite is, the more preferable, and it is necessary to control the average crystal particle size to 3.0 μm or less. There is. More preferably, it is less than 2.7 μm, but the lower limit is not particularly limited. Since it is difficult to make it less than 0.5 μm in the current actual operation, 0.5 μm is a practical lower limit.
[0045]
　The average crystal grain size of old austenite is measured as follows.
[0046]
　First, the hot stamp molded product is heat-treated at 540 ° C. for 24 hours. This promotes corrosion of the old austenite grain boundaries. The heat treatment may be performed by heating in a furnace or energizing, and the heating rate is 0.1 to 100 ° C./s and the cooling rate is 0.1 to 150 ° C./s.
[0047]
　A cross section perpendicular to the plate surface is cut out from the central portion of the hot stamped molded product after the heat treatment, the measurement surface is polished using silicon carbide paper of # 600 to # 1500, and then diamond powder having a particle size of 1 to 6 μm is mixed with alcohol or the like. Finish with a mirror surface using a diluted solution of No. 1 or a liquid dispersed in pure water.
[0048]
　Next, the observation surface is immersed in a 3-4% sulfuric acid-alcohol (or water) solution for 1 minute to reveal the old austenite grain boundaries. At this time, the corrosion work is carried out in the exhaust treatment device, and the temperature of the work atmosphere is normal temperature.
[0049]
　The corroded sample is washed with acetone or ethyl alcohol, dried, and subjected to scanning electron microscopy. The scanning electron microscope used shall be equipped with a two-electron detector.
[0050]
　In a vacuum of 9.6 × 10-5 or less, the sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13, and the sample is placed at the 1/8 to 3/8 position centered on the 1/4 position of the sample thickness. Take a secondary electron image of the range. The shooting magnification is 4000 times based on a screen having a width of 386 mm and a height of 290 mm, and the number of shooting fields of view is 10 or more.
[0051]
　In the photographed secondary electron image, the old austenite grain boundaries are imaged as bright contrast. The average value of the shortest diameter and the longest diameter of the old austenite grains included in the observation field is calculated and used as the average crystal grain size. The above operation is performed on all the old austenite grains except for the old austenite grains whose entire grains are not included in the shooting field of view, such as the edge of the shooting field of view, and the average crystal grain size in the shooting field of view is obtained. The average crystal grain size is a value obtained by dividing the calculated total grain size by the total number of former austenite grains whose grain size has been measured. This operation is performed for each field of view in which the images are taken, and the average crystal grain size of the old austenite is calculated.
[0052]
　"The grain boundary solid solution ratio Z defined by the formula (1) is 0.3 or more."
[0053]
　Z = mass% of 1 or 2 types of Nb and Mo at the grain boundary / mass% of 1 or 2 types of Nb and Mo at the time of dissolution ... (1)
[0054]
　The grain boundary solid solution ratio Z defined by the above formula (1) is an important tissue factor for ensuring excellent shock absorption capacity, and is an index adopted by the present inventors to evaluate the shock absorption capacity. is there. When Nb and / or Mo is solid-solved at the grain boundaries, P is less likely to segregate at the grain boundaries and the binding force at the grain boundaries is increased, so that the embrittlement strength of the grain boundaries is increased and the shock absorption capacity is improved. If the grain boundary solid solution ratio Z is less than 0.3, the grain boundary strengthening effect of Nb and / or Mo cannot be sufficiently obtained, and the required impact absorption capacity cannot be obtained. Therefore, the grain boundary solid solution ratio cannot be obtained. Z is 0.3 or more. It is preferably 0.4 or more. The upper limit is not particularly limited, but theoretically 1.0 is the upper limit.
[0055]
　The grain boundary solid solution ratio Z is measured as follows.
[0056]
　A test piece having the dimensions shown in FIG. 1 is prepared from the central portion of the hot stamp molded product. At this time, the front and back surfaces of the test piece are removed by mechanical grinding in equal amounts so that the plate thickness is 1.2 mm. The notch at the center of the test piece is inserted by a wire cutter having a thickness of 1 mm, and the joint at the bottom of the notch is controlled from 100 μm to 200 μm.
[0057]
　Next, the test piece is immersed in a 20% -ammonium thiocyanate solution for 72 to 120 hr.
[0058]
　Galvanize the front and back surfaces of the test piece within 0.5 hr after the immersion is completed.
[0059]
　After plating, it is subjected to Auger electron emission spectroscopic analysis within 1.5 hr. The type of apparatus for performing Auger electron emission spectroscopic analysis is not particularly limited. The test piece is set in the analyzer and broken from the notch of the test piece in a vacuum of 9.6 × 10-5 or less to expose the old austenite grain boundaries. The exposed old austenite grain boundaries are irradiated with an electron beam at an accelerating voltage of 1 to 30 kV, and the mass% (concentration) of Nb and / or Mo at the grain boundaries is measured. Measurements are performed at 10 or more former austenite grain boundaries. Measurements should be completed within 30 minutes of destruction to prevent grain boundary contamination.
[0060]
　The average value of the mass% (concentration) of the obtained Nb and / or Mo is calculated, and the value divided by the mass% of the added Nb and / or Mo is defined as the grain boundary solid solution ratio Z.
[0061]
　"More than 90% of the microstructure area ratio is one or more of lower bainite, martensite and tempered martensite."
[0062]
　In order for the hot stamped product to obtain a tensile strength of 1500 MPa or more, the microstructure needs to contain martensite or tempered martensite having an area ratio of 90% or more. It is preferably 94% or more. From the viewpoint of ensuring tensile strength, the microstructure may be lower bainite. The structure having an area ratio of 90% or more may be one of lower bainite, martensite and tempered martensite, or a mixed structure thereof.
[0063]
　The remainder of the microstructure is not particularly specified, and examples thereof include upper bainite, retained austenite, and pearlite.
[0064]
　The area ratio of lower bainite, martensite, and tempered martensite is measured as follows.
[0065]
　A cross section perpendicular to the plate surface is cut out from the center of the hot stamped molded product, and the measurement surface is polished using # 600 to # 1500 silicon carbide paper. Finish with a mirror surface using a liquid dispersed in pure water.
[0066]
　Immerse in a 1.5-3% nitric acid-alcohol solution for 5-10 seconds to reveal high-inclined grain boundaries. At this time, the corrosion work is carried out in the exhaust treatment device, and the temperature of the work atmosphere is normal temperature.
[0067]
　The corroded sample is washed with acetone or ethyl alcohol, dried, and subjected to scanning electron microscopy. The scanning electron microscope used shall be equipped with a two-electron detector. In a vacuum of 9.6 × 10-5 or less, the sample is irradiated with an electron beam at an acceleration voltage of 10 kV and an irradiation current level of 8, and the sample is placed at 1/8 to 3/8 positions centered on the 1/4 position of the sample thickness. Take a secondary electron image of the area. The shooting magnification is 10000 times based on a screen having a width of 386 mm and a height of 290 mm, and the number of viewing fields is 10 or more.
[0068]
　In the photographed secondary electron image, the crystal grain boundaries and the carbides are imaged as a bright contrast, so that the structure can be easily determined by the positions of the crystal grain boundaries and the carbides. When carbides are formed inside the crystal grains, it is tempered martensite or lower bainite, and the structure in which no carbides are observed inside the crystal grains is martensite.
[0069]
　On the other hand, the structure in which carbides are formed at the grain boundaries is upper bainite or pearlite.
[0070]
　Since the crystal structure of retained austenite is different from that of the microstructure, the same field of view as the position where the secondary electron image is imaged is measured by electron backscatter diffraction. The scanning electron microscope used shall be equipped with a camera capable of electron backscatter diffraction. A sample is irradiated with an electron beam at an accelerating voltage of 25 kV and an irradiation current level of 16 in a vacuum of 9.6 × 10-5 or less to perform measurement, and a face-centered cubic lattice map is created from the obtained measurement data.
[0071]
　A mesh with an interval of 2 μm is created on a photograph taken at 10000 times with a screen of 386 mm in width × 290 mm in length as a reference, and microstructures located at the intersection of the meshes are selected. The value obtained by dividing the number of intersections of each tissue by all the intersections is defined as the area ratio of the microstructure. This operation is performed in 10 fields of view, the average value is calculated, and the area ratio of the microstructure is used.
[0072]
　Next, a form of a hot stamping molded product according to the present invention and a manufacturing method for obtaining a hot stamping steel plate used for manufacturing the hot stamping molded product will be described.
[0073]
　
[0074]
(1) Continuous casting process
　A molten steel having the above-mentioned chemical composition is made into steel pieces (slabs) by a continuous casting method. In this continuous casting step, the amount of molten steel cast per unit time is preferably 6 tons / minute or less. If the casting amount (casting speed) of molten steel per unit time exceeds 6 tons / minute during continuous casting, the microsegregation of Mn increases and the nucleation amount of precipitates mainly composed of Mo and Nb increases. .. It is more preferable that the casting amount is 5 ton / min or less. The lower limit of the casting amount is not particularly limited, but is preferably 0.1 ton / min or more from the viewpoint of operating cost.
[0075]
　(2) Hot rolling process The
　above-mentioned steel pieces are hot-rolled to obtain a steel plate. At that time, hot rolling is completed in a temperature range of A3 transformation temperature + 10 ° C. or higher and A3 transformation temperature + 200 ° C. or lower defined by the formula (2), and the final stage reduction ratio at that time is set to 12% or higher for finish rolling. Cooling is started within 1 second after the completion, the temperature range from the finish rolling end temperature to 550 ° C. is cooled at a cooling rate of 100 ° C./s or more, and the mixture is wound at a temperature of less than 500 ° C.
[0076]
　A3 transformation temperature = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo ・ ・ ・ ・ Equation (2)
[0077]
　By setting the finish rolling temperature to A3 transformation temperature + 10 ° C. or higher, recrystallization of austenite is promoted. As a result, the formation of small tilt angle grain boundaries in the crystal grains is suppressed, and the precipitation sites of Nb and Mo can be reduced. Further, by reducing the precipitation sites of Nb and Mo, the consumption of C can be suppressed, so that the number density of carbides can be increased in a later step. Preferably, the A3 transformation temperature is + 30 ° C. or higher.
[0078]
　By setting the finish rolling temperature to A3 transformation temperature + 200 ° C. or lower, excessive grain growth of austenite is suppressed. By finish rolling in the temperature range of A3 transformation temperature + 200 ° C. or less, recrystallization of austenite is promoted, and excessive grain growth does not occur, so that fine carbides can be obtained in the winding step. Preferably, the A3 transformation temperature is + 150 ° C. or lower.
[0079]
　Recrystallization of austenite is promoted by setting the rolling reduction ratio of finish rolling to 12% or more. As a result, the formation of small tilt angle grain boundaries in the crystal grains is suppressed, and the precipitation sites of Nb and Mo can be reduced. Preferably, it is 15% or more.
[0080]
　Cooling is started within 1 second, preferably within 0.8 seconds after the end of finish rolling, and the temperature range from the end of finish rolling to 550 ° C. is cooled at a cooling rate of 100 ° C./s or more to obtain Nb and Nb. The residence time in the temperature range where the precipitation of Mn is promoted can be reduced. As a result, the precipitation of Nb and Mo in austenite can be suppressed, and the solid solution amount of Nb and Mo in the austenite grain boundary increases.
[0081]
　By setting the winding temperature to less than 500 ° C., the above effect is enhanced, Mn concentration in carbides is suppressed, easily soluble fine carbides are generated, and high-density dislocations are introduced into the steel. To do. It is preferably less than 480 ° C. The lower limit is not specified, but since it is difficult to wind up below room temperature in actual operation, room temperature is the lower limit.
[0082]
　(3) Formation of Plating Layer A
　plating layer may be formed on the surface of the steel sheet for the purpose of improving corrosion resistance and the like. The plating layer may be either an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include an electrogalvanizing layer and an electric Zn—Ni alloy plating layer. The hot-dip galvanizing layer includes a hot-dip galvanizing layer, an alloyed hot-dip galvanizing layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip Zn-Al-Mg-Si alloy. A plating layer and the like are exemplified. The amount of adhesion of the plating layer is not particularly limited and may be a general amount of adhesion.
[0083]
　(4) Other Steps In
　the production of the hot stamping steel sheet, known production methods such as pickling, cold rolling, and temper rolling may be included.
[0084]
　
[0085]
　In the hot stamping molded product of the present invention, a steel plate for hot stamping is heated and held in a temperature range of 500 ° C. or higher and A3 point or lower at an average heating rate of 100 ° C./s or higher and lower than 200 ° C./s, and then hot stamping is performed. It is manufactured by molding and after molding, the molded product is cooled to room temperature.
[0086]
　Further, in order to adjust the strength, a part or all of the hot stamped molded product may be tempered at a temperature of 200 ° C. or higher and 500 ° C. or lower.
[0087]
　By heating and holding the temperature range of 500 ° C. or higher and A3 point or lower at an average heating rate of 100 ° C./s or higher and lower than 200 ° C./s and hot stamping, both easily soluble fine carbides and high-density dislocations are removed. The nucleation site of the former austenite can be used, and the average grain size of the former austenite can be controlled to 3 μm or less. Furthermore, it also contributes to suppressing the precipitation of NbC and MoC during heating and increasing the solid solution ratio of one or two types of Nb and Mo at the grain boundaries of the former austenite.
[0088]
　The average heating rate is preferably 120 ° C./s or higher. If the average heating rate exceeds 200 ° C./s, the transformation to austenite is promoted without the dissolution of carbides being completed, which causes deterioration of toughness. Therefore, the upper limit is 200 ° C./s. It is preferably less than 180 ° C./s.
[0089]
　The holding temperature at the time of hot stamping is preferably A3 point + 10 ° C. or higher and A3 point + 150 ° C. or lower. The cooling rate after hot stamping is preferably 10 ° C./s or higher.
Example
[0090]
　Next, an example of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is described in this one condition example. It is not limited. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0091]
　Steel sheets for hot stamping are hot-rolled and cold-rolled under the conditions shown in Tables 2-1 to 2-3 on steel pieces manufactured by casting molten steel with the composition shown in Tables 1-1 to 1-3. Then, the obtained steel sheet for hot stamping was subjected to the heat treatments shown in Tables 2-1 to 2-3 to perform hot stamping to produce a molded product.
[0092]
　Tables 3-1 to 3-3 show the microstructure and mechanical properties of the hot stamped product.
[0093]
[Table 1-1]

[0094]
[Table 1-2]

[0095]
[Table 1-3]

[0096]
[Table 2-1]

[0097]
[Table 2-2]

[0098]
[Table 2-3]

[0099]
[Table 3-1]

[0100]
[Table 3-2]

[0101]
[Table 3-3]

[0102]
　Further, the tensile strength of the hot stamp molded product was measured according to the test method described in JIS Z 2241 by preparing the No. 5 test piece described in JIS Z 2201. As an index of shock absorption capacity, toughness was evaluated by Charpy impact test. A sub-size Charpy impact test was performed at −100 ° C., and a case where the brittle fracture surface ratio was less than 30% was accepted.
[0103]
　It was confirmed that the hot stamp molded product of the present invention has excellent properties such as a tensile strength of 1500 MPa or more and a brittle fracture surface ratio of less than 30%, which is an index of toughness. On the other hand, in the case where the chemical composition and the production method were not appropriate, the target characteristics could not be obtained.
The scope of the claims
[Claim 1]
　Ingredient composition is mass%,
　　C: 0.15% or more, less than 0.35%,
　　Si: 0.005% or more, 0.25% or less,
　　Mn: 0.5% or more, 3.0% or less,
　　sol. Al: 0.0002% or more, 3.0% or less,
　　Cr: 0.05% or more, 1.00% or less,
　　B: 0.0005% or more, 0.010% or less,
　　Nb: 0.01% or more, 0.15% or less,
　　Mo: 0.005% or more, 1.00% or less,
　　Ti: 0% or more, 0.15% or less,
　　Ni: 0% or more, 3.00% or less,
　　P: 0.10% Hereinafter,
　　S: 0.10% or less and
　　N: 0.010% or less
are contained, the balance is Fe and unavoidable impurities, and the
　microstructure contains bainite having an average crystal grain size of 3 μm or less, and further. , Lower bainite, martensite and tempered martensite at least 90% in area ratio,
　Z = (mass% of 1 or 2 of Nb and Mo at grain boundaries) / (Nb and at dissolution)
A hot stamped molded product having a defined grain boundary solid solution ratio Z of 0.3 or more (mass% of 1 or 2 types of Mo) .
[Claim 2]
　The hot stamp molded product according to claim 1, which has a plating layer.