Title of invention: Steel plate for hot stamping
Technical field
[0001]
　The present invention relates to a steel plate for hot stamping, which is used for structural members and reinforcing members of automobiles and structures that require strength, and is particularly used as a material for a hot stamping molded body having excellent strength and bending deformability.
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. Since hot stamping is hardened in a mold at the same time as press working, it is attracting attention as a technology that achieves both molding into automobile members and ensuring strength.
[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 a steel sheet for hot stamping is annealed and Mn and Cr are concentrated in the carbide to make it a carbide that is difficult to dissolve. A technique for suppressing growth and granulating 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 higher strength or bending deformability than before.
[0010]
　In view of the problems of the prior art, it is an object of the present invention to secure better strength or bending deformability in a hot stamped body of a high-strength steel plate, and to provide a hot stamping steel plate that solves the problem. With the goal.
Means to solve problems
[0011]
　The present inventors have diligently studied a method for solving the above problems. As a result, it was found that by setting the particle size of the old austenite of the hot stamp molded product to 3 μm or less, excellent strength can be obtained as compared with the conventional one.
[0012]
　Then, in order to reduce the particle size of the old austenite of the hot stamped product to 3 μm or less, the number density of cementite or epsilon carbide should be 1 × 10 16 / cm 3 or more in the steel plate before molding , and further, Nb and Mo It has been found that one or two species may be dissolved in the old austenite grain boundaries to increase the embrittlement strength of the grain boundaries.
[0013]
　Further, in the steel plate for hot stamping, the texture memory of austenite and martensite is controlled by controlling the X-ray random intensity ratio of {112} <111>, which is the crystal orientation of the crystal grains of lower bainite or martensite or tempered martensite. It has been found that the effect produces a crystal orientation having a high effect of suppressing crack growth in the hot stamped compact, and an excellent bending deformation ability can be obtained in the hot stamped compact.
[0014]
　The invention of the present application has been further studied based on the above findings, and the gist thereof is as follows.
[0015]
　(1) Ingredient composition is C: 0.35% or more, 0.75% or less, 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% or less , S: 0.10% or less, N: 0.010% or less, the balance is Fe and unavoidable impurities, and the microstructure is an area ratio of at least one of lower bainite, martensite and tempered martensite. 90% or more, and 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) The ratio Z is 0.4 or more, the X-ray random intensity ratio of {112} <111> of the crystal grains constituting the lower bainite, martensite, or tempered martensite is 2.8 or more, and the grain size is 2.8 or more. A steel plate for hot stamping, wherein the total number density of cementite and epsilon carbides of 50 nm or less is 1 × 10 16 / cm 3 or more.
[0016]
　(2) The hot stamping steel sheet according to (1) above, which has a plating layer.
The invention's effect
[0017]
　According to the present invention, it is possible to provide a hot stamping steel sheet which is a material of a hot stamping molded product having excellent strength or bending deformability.
A brief description of the drawing
[0018]
[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
[0019]
　The features of the present invention are that the number density of cementite or epsilon carbide is 1 × 10 16 / cm 3 or more, and one or two of Nb and Mo are dissolved in the old austenite grain boundaries to embrittle the grain boundaries. It is to increase the strength. Further, it is to control the X-ray random intensity ratio of {112} <111>, which is the crystal orientation of the crystal grains of the lower bainite or martensite or tempered martensite of the steel sheet. As a result of diligent studies, the present inventors have found that the above-mentioned structure can be obtained by the following method.
[0020]
　As a first step, the casting amount of molten steel 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.
[0021]
　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 at 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.
[0022]
　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.
[0023]
　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.
[0024]
　Furthermore, by controlling the coil winding conditions, Mn concentration in the carbide is suppressed, fine carbides that are easily dissolved are generated, and further, high-density dislocations are introduced into the steel to introduce the strength of austenite. Is preferentially produced, although it is an advantageous crystal orientation for alleviating the stress generated by the transformation during phase transformation from austenite to lower bainite or martensite or tempered martensite. As a result, the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled.
[0025]
　These hot stamping steel sheets exhibit different characteristics by controlling the heating rate in the hot stamping process.
[0026]
　Hereinafter, the hot stamping steel sheet of the present invention and its manufacturing method will be described. First, the reason for limiting the component composition constituting the hot stamping steel sheet of the present invention will be described. Hereinafter,% related to the component composition means mass%.
[0027]
　"C: 0.35% or more, 0.75% or less"
　C is an important element for the hot stamp molded product to obtain a tensile strength of 2000 MPa or more. If it is less than 0.35%, martensite is soft and it is difficult to secure a tensile strength of 2000 MPa or more. Therefore, C is set to 0.35% or more. It is preferably 0.37% or more. The upper limit is 0.75% in consideration of the balance between the required strength and the suppression of early fracture.
[0028]
　"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 impact absorption capacity of the hot stamp molded product 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 particle size of the old austenite in the hot stamp molded product cannot be controlled to 3 μm, so the upper limit is 0.25%. And. It is preferably 0.22% or less.
[0029]
　"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 for the hot stamped product to secure a tensile strength of 2000 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 particle size of the old austenite in the hot stamp molded product cannot be controlled to 3 μm or less. The upper limit is%. It is preferably 2.5% or less.
[0030]
　"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 a coarse oxide having a diameter of 5 μm or more is generated, which causes 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, and more preferably 0.5% or less.
[0031]
　"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 for the hot stamp molded product to secure a tensile strength of 2000 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 in the hot stamp molded product cannot be controlled to 3 μm or less. The upper limit is%. Preferably, it is 0.8% or less.
[0032]
　"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 for the hot stamp molded product to secure a tensile strength of 2000 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 particle size of the old austenite in the hot stamp molded product cannot be controlled to 3 μm or less. The upper limit is%. Preferably, it is 0.007% or less.
[0033]
　"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. Furthermore, the strength of austenite can be increased by dissolving Nb and Mo in solid solution in austenite immediately after finish rolling and further controlling the coil winding conditions, from austenite to lower bainite or martensite or tempered martensite. At the time of phase transformation with, the crystal orientation is advantageous for alleviating the stress generated by the transformation, but it is preferentially generated. As a result, the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled. 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.
[0034]
　"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. Furthermore, the strength of austenite can be increased by dissolving Nb and Mo in solid solution in austenite immediately after finish rolling and further controlling the coil winding conditions, from austenite to lower bainite or martensite or tempered martensite. At the time of phase transformation with, the crystal orientation is advantageous for relaxing the stress generated by the transformation, but it is preferentially generated. As a result, the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled. 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.
[0035]
　"Ti: 0% or more, 0.15% or less"
　Ti is not an essential element, but it may be added as needed 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 excess of 0.15%, coarse carbides and nitrides having a diameter of 5 μm or more are formed and early fracture is caused, so the content is 0.15% or less. It is preferably 0.12% or less.
[0036]
　"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.
[0037]
　"P: 0.10% or less"
　P is an impurity element, which easily segregates at the grain boundaries and reduces the embrittlement strength of the grain boundaries. If it exceeds 0.10%, the embrittlement strength of the grain boundaries is remarkably lowered and early fracture is caused, so 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 cost of removing P is significantly increased, which is economically disadvantageous. Therefore, 0.0001% is a practical lower limit on the practical steel sheet.
[0038]
　"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.
[0039]
　"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.
[0040]
　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 product of the present invention.
[0041]
　Next, the reason for limiting the microstructure constituting the hot stamping steel sheet of the present invention will be described.
[0042]
　"More than 90% of the microstructure area ratio is one or more of lower bainite, martensite and tempered martensite."
[0043]
　In order for the hot stamped product to obtain a tensile strength of 1500 MPa or more, the microstructure of the hot stamping steel sheet 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 balance is not particularly specified, and examples thereof include upper bainite, retained austenite, and pearlite.
[0044]
　The area ratio of lower bainite, martensite, and tempered martensite is measured as follows.
[0045]
　A cross section perpendicular to the plate surface is cut out from the central part of the hot stamping steel plate, 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 diluted with alcohol or the like. Finish with a mirror surface using a liquid or a liquid dispersed in pure water.
[0046]
　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.
[0047]
　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 the 1/8 to 3/8 position 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 shooting fields of view is 10 or more.
[0048]
　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.
[0049]
　On the other hand, the structure in which carbides are formed at the grain boundaries is upper bainite or pearlite.
[0050]
　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. In a vacuum of 9.6 × 10-5 or less, the sample is irradiated with an electron beam at an accelerating voltage of 25 kV and an irradiation current level of 16, and measurement is performed, and a face-centered cubic lattice map is created from the obtained measurement data.
[0051]
　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 surface integral 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.
[0052]
　"The grain boundary solid solution ratio Z defined by the formula (1) is 0.4 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 of the hot stamped product is less than 0.4, the grain boundary strengthening effect of Nb and / or Mo cannot be sufficiently obtained, and the required shock absorbing ability cannot be obtained. When the hot stamping steel sheet is subjected to hot stamping, the grain boundary solid solution amounts of Nb and Mo are reduced by the heat treatment, so the grain boundary solid solution ratio Z is set to 0.4 or more. It is preferably 0.5 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 stamping steel plate. 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]
　"The X-ray random intensity ratio of {112} <111> of the crystal grains constituting the lower bainite or martensite or tempered martensite is 2.8 or more."
[0062]
　In the hot stamping steel sheet, if the X-ray random intensity ratio of {112} <111> of the crystal grains constituting the lower bainite, martensite, or tempered martensite is less than 2.8, the hot stamped product cracks. A crystal orientation having a high growth suppressing effect cannot be generated, and an excellent bending deformation ability cannot be obtained. Therefore, the X-ray random intensity ratio is set to 2.8 or more. The X-ray random intensity ratio is preferably 3.0 or more. The upper limit is not particularly set, but 15.0 is the actual upper limit because it is difficult to make it 15.0 or more in actual operation.
[0063]
　Next, a method of calculating the metal structure will be described.
[0064]
　A sample is cut out from the center of the hot stamping steel plate so that a cross section perpendicular to the surface (thick cross section) can be observed. After polishing the measurement surface with # 600 to # 1500 silicon carbide paper, a mirror surface is finished using a diluted solution such as alcohol or a liquid in which diamond powder having a particle size of 1 to 6 μm is dispersed in pure water.
[0065]
　Next, finish polishing is performed using a standard colloidal silica suspension (particle size 0.04 μm). The polished sample is washed with acetone or ethyl alcohol, dried, and set in a scanning electron microscope. It is assumed that the scanning electron microscope to be used is equipped with an EBSD detector (DVC5 type detector manufactured by TSL).
[0066]
　Crystal orientation information is obtained by EBSD measurement at measurement intervals of 0.2 μm in a range of 500 μm in the plate thickness direction and 1000 μm in the rolling direction at the 3/8 to 5/8 position of the sample plate thickness. The measurement conditions are a vacuum level of 9.6 × 10-5 or less, an acceleration voltage of 15 kV, an irradiation current level of 13, a binning size of 8 × 8, and an exposure time of 62 seconds.
[0067]
　The measurement data is analyzed using the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, and the X-ray random intensity ratio of {112} <111> is calculated. It is a parameter that is installed in the software using the "Texture" function "crystalline orientation distribution function" function, phi 2 to draw the crystal orientation distribution function = 45 ° cross section. From the drawn image, the X-ray random intensity ratio at the {112} <111> pole position is read.
[0068]
　"The total number density of cementite and epsilon carbides with a particle size of 50 nm or less is 1 x 10 16 pieces / cm 3 or more." The
　total number density of cementite and epsilon carbides with a particle size of 50 nm or less is 1 x 10 16 pieces / cm. If it is 3 or more, the finely dispersed carbide becomes a reverse transformation site of austenite, so that the old austenite grains of the hot stamped product can be made finer. If the number density is less than 1 × 10 16 pieces / cm 3 , the effect cannot be obtained, so the lower limit is 1 × 10 16 pieces / cm 3 . Preferably, it is 3 × 10 16 pieces / cm 3 . The upper limit is not specified, but the upper limit is 1000 × 10 16 pieces / cm 3 in consideration of the required strength and the balance of early fracture suppression . If the steel sheet is manufactured under the manufacturing conditions specified in the present application, the carbides produced are mainly cementite and epsilon carbides.
[0069]
　Next, a method of calculating the metal structure will be described.
[0070]
　A sample is cut out from the hot stamping steel plate so that a cross section perpendicular to the surface (thickness cross section) can be observed. After polishing the measurement surface with # 600 to # 1500 silicon carbide paper, a mirror surface is finished using a diluted solution such as alcohol or a liquid in which diamond powder having a particle size of 1 μm to 6 μm is dispersed in pure water.
[0071]
　Next, electric field etching by the SPEED method using a water-insoluble electric field solution described in "Fumio Kurosawa, Isamu Taguchi, Ryutaro Matsumoto, Journal of the Japan Institute of Metals, 43, 1068 (1979)" is performed, and fine carbides can be easily observed. Prepare the sample so as to. This method utilizes the fact that carbon steel and cementite and epsilon carbides have different decomposition potentials, and electrolyzes at a potential at which only the base metal decomposes, so that the carbides can be easily observed. By using a water-insoluble electrolyte, decomposition of water-soluble cementite and epsilon carbides is suppressed, so that it is suitable for measuring the dimensions and number density of fine carbides.
[0072]
　The observation surface of the sample is immersed in an acetylacetone-based electrolytic solution, and electrolysis is performed for 2 seconds at an electrolytic potential of 300 mV. The sample after the electric field is washed with acetone or ethyl alcohol, dried, and set in a scanning electron microscope. As the scanning electron microscope to be used, a model equipped with a secondary electron detector is used. In a vacuum of 9.6 x 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 thickness is 386 mm x 290 mm at the 3/8 to 5/8 positions. 10 fields of view with a magnification of 30,000 times are observed with reference to the screen of.
[0073]
　The number of cementite and epsilon carbides having a particle size (long axis length) of 50 nm or less included in the observation field is measured. A value obtained by dividing the number of the carbides contained in one visual field by the area of ​​the observation visual field is calculated. The same operation is performed in 10 fields of view, and the average value of all fields of view is taken as the number density of cementite and epsilon carbides.
[0074]
　Next, a form of a manufacturing method for obtaining a hot stamping steel sheet according to the present invention will be described.
[0075]
　
[0076]
(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 process, the amount of molten steel cast per unit time is set to 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.
[0077]
　(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 + 30 ° 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./sec or more, and the mixture is wound at a temperature of less than 500 ° C.
[0078]
　A3 transformation temperature = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo ・ ・ ・ ・ Equation (2)
[0079]
　The recrystallization of austenite is promoted by setting the finish rolling temperature to A3 transformation temperature + 30 ° C. or higher. 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, the A3 transformation temperature is + 50 ° C. or higher.
[0080]
　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.
[0081]
　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.
[0082]
　Cooling is started within 1 second, preferably within 0.8 seconds after the finish rolling, and the temperature range from the finish rolling end temperature to 550 ° C. is cooled at a cooling rate of 100 ° C./sec or higher to obtain Nb and 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.
[0083]
　By setting the winding temperature to less than 500 ° C., the above effect is enhanced, Mn concentration in the carbide is suppressed, easily soluble fine carbide is generated, and high-density dislocations are introduced into the steel. To do. It is preferably less than 480 ° C. When the winding temperature exceeds 500 ° C., the total number density of cementite and epsilon carbide having a particle size of 50 nm or less does not become 1 × 10 16 pieces / cm 3 or more. 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.
[0084]
　Immediately after finish rolling, Nb and Mo are solid-solved in austenite, and Nb and Mo are transformed from austenite in which Nb and Mo are solid-solved into lower bainite, martensite, or tempered martensite. Preferentially generates an advantageous crystal orientation in order to relieve the stress generated by the transformation. Therefore, as described above, cooling is started within 1 second after the finish rolling, and the temperature is from the finish rolling end temperature to 550 ° C. By cooling the temperature range at a cooling rate of 100 ° C./sec or higher, the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled.
[0085]
　(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.
[0086]
　(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.
[0087]
　
　Next, a form of a manufacturing method for obtaining a hot stamping molded product using the hot stamping steel sheet according to the present invention will be described. The method for obtaining the hot stamp molded product is not limited to the following forms.
[0088]
　(Manufacturing method A) Manufacturing method for obtaining a hot stamped molded product with excellent strength A
　steel sheet for hot stamping is held by heating it in a temperature range of 500 ° C. or higher and A3 points or lower at an average heating rate of 100 ° C./s or higher and lower than 200 ° C./s. After that, hot stamp molding is performed, and after molding, the molded product is cooled to room temperature. 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.
[0089]
　By heating 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, both easily soluble fine carbides and high-density dislocations become nucleation sites of former austenite. , The average particle size of the old 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. Preferably, it is 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.
[0090]
　The holding temperature at the time of hot stamping is preferably A3 point + 50 ° C. or higher and A3 point + 150 ° C. or lower. The cooling rate after hot stamping is preferably 10 ° C./s or higher.
[0091]
　(Manufacturing method B: A manufacturing method for obtaining a hot stamped compact having excellent bending deformation)
　A3 is a steel sheet for hot stamping as it is, a steel sheet obtained by cold rolling the steel sheet, or a steel sheet obtained by plating the steel sheet. After heating and holding at an average speed of less than 100 ° C./s above the point, hot stamping is performed, and after molding, the molded product is cooled to room temperature. 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.
[0092]
　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
[0093]
　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.
[0094]
　A steel piece produced by casting molten steel having the composition shown in Table 1 was hot-rolled as shown in Table 2 to obtain a steel sheet for hot stamping. Regarding the obtained steel plate for hot stamping, the area ratio of lower bainite, martensite and tempered martensite, the grain boundary solid dissolution ratio of Nb and Mo, and the crystals constituting the lower bainite or martensite or tempered martensite by the above-mentioned method. The random X-ray intensity ratio of {112} <111> of the grains and the number density of cementite and epsilon carbides having a grain size of 50 nm or less were measured.
[0095]
　Further, using the obtained steel sheet for hot stamping, cold rolling and plating were performed under the conditions shown in Table 3 to prepare a hot stamped compact. In the heat treatment at the time of hot stamping, the average heating rate in the temperature range of 500 ° C. or higher and A3 point or lower was performed at various rates.
[0096]
[Table 1-1]

[0097]
[Table 1-2]

[0098]
[Table 1-3]

[0099]
[Table 2-1]

[0100]
[Table 2-2]

[0101]
[Table 2-3]

[0102]
[Table 3-1]

[0103]
[Table 3-2]

[0104]
[Table 3-3]

[0105]
　The tensile strength of the sample prepared by setting the average heating rate in the temperature range of 500 ° C. or higher and A3 point or lower to 100 ° C./s or higher was measured, and the impact absorption capacity was further evaluated.
[0106]
　The tensile strength of the sample prepared by setting the average heating rate in the temperature range of 500 ° C. or higher and A3 point or lower to less than 100 ° C./s was measured, and the bending deformability was further evaluated.
[0107]
　In addition, the impact absorption capacity was evaluated based on the presence or absence of early fracture, and materials that did not undergo early fracture were accepted according to the following evaluation criteria. Excellent shock absorption means that the amount of energy absorbed at the time of collision is large. That is, the integral value in the stress-strain curve is large, which can be evaluated by not breaking early (breaking after reaching the maximum stress).
[0108]
　When the value obtained by dividing the maximum strength obtained in the tensile test by 3.3 times the Vickers hardness of the material was 0.85 or more, it was judged that the early fracture was suppressed. The Vickers hardness of the material was measured by the following method.
[0109]
　A cross section perpendicular to the plate surface is cut out from the hot stamp molded product, the measurement surface is polished using # 600 to # 1500 silicon carbide paper, and then diamond powder having a particle size of 1 to 6 μm is mixed with a diluted solution such as alcohol or pure. Use a liquid dispersed in water to make a mirror finish. Using a Vickers hardness tester, 10 points were measured at a plate thickness of 1/4 with a load of 1 kgf and a measurement interval of 3 times or more the indentation, and the average value was taken as the hardness of the steel plate.
[0110]
　The bending deformability was evaluated under the following measurement conditions based on the VDA standard (VDA238-100) specified by the German Automobile Manufacturers Association. In the present invention, the displacement at the maximum load obtained in the bending test is converted into an angle based on the VDA standard, the maximum bending angle is obtained, and the material having the maximum bending angle of 50 ° or more is accepted.
[0111]
　　Specimen dimensions: 60 mm (rolling direction) x 30 mm (direction perpendicular to rolling), plate thickness 1.0 mm
　　Bending ridge: direction perpendicular to rolling
　　Test method: roll support, punch pushing
　　roll diameter: φ30 mm
　　Punch shape: tip R = 0 .4mm
　　Distance between rolls: 2.0 × 1.0 (mm) + 0.5mm
　　Pushing speed: 20mm / min
　　Testing machine: SHIMADZU AUTOGRAPH 20kN
[0112]
　It was confirmed that the steel sheet for hot stamping of the present invention has a tensile strength of 2000 MPa or more and has excellent bending deformability. 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: 0.35% or more, 0.75% or less,
　　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% or less ,
　　S: 0.10% or less,
　　N: 0.010% or less
, the balance is Fe and unavoidable impurities, and the
　microstructure is the area ratio of at least one of lower bainite, martensite and tempered martensite. 90% or more, and
　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) defined grain boundary solid dissolution The ratio Z is 0.4 or more, and
　the X-ray random intensity ratio of {112} <111> of the crystal grains constituting the lower bainite, martensite, or tempered martensite is 2.8 or more.
　A steel sheet for hot stamping, wherein the total density of cementite and epsilon carbides having a particle size of 50 nm or less is 1 × 10 16 pieces / cm 3 or more
.
[Claim 2]
　The hot stamping steel sheet according to claim 1, further comprising a plating layer.