TWI663265B - Hot stamping steel plate - Google Patents

Abstract

A steel plate for hot stamping, which can be used as a blank for a hot stamped formed body having excellent strength or bending deformation ability. The steel plate for hot stamping is characterized in that it has a predetermined composition and the microstructure includes 90 in terms of area ratio. At least one of the above toughened iron, Asada loose iron and tempered Asada loose iron; {112} <111> X-rays constituting the grains of the lower toughened iron, Asada loose iron or tempered Asada loose iron The random intensity ratio is 2.8 or more; the total number density of Schiff carbon iron and ε carbides with a particle size of 50 nm or less is 1 × 10 ¹⁶ pieces / cm ³ or more; the Schiff carbon iron with a particle size of 50 nm or less in the microstructure Or the number density of ε carbides is 1 × 10 ¹⁶ pieces / cm ³ or more; and the grain boundary solid solution ratio Z is 0.4 or more. The aforementioned grain boundary solid solution ratio Z is defined as Z = (Nb and Mo in grain boundaries 1% or 2% by mass) / (1% or 2% by mass of Nb and Mo when dissolved).

Description

Hot stamping steel plate

The present invention relates to a hot stamping steel sheet used for structural members or reinforcing members of automobiles or structures that require strength, and particularly relates to hot stamping that can be used as a blank for a hot stamped formed body having excellent strength and bending deformation ability. With steel plate.

In recent years, from the viewpoints of environmental protection and resource conservation, the weight of automobile bodies has been continuously demanded. Therefore, the application of high-strength steel plates to automobile components has continued to accelerate. However, since the formability deteriorates with the increase in the strength of the steel sheet, for high-strength steel sheets, the formability of members having complex shapes becomes a problem.

In order to solve the above-mentioned problems, the application of hot embossing after press forming after heating a steel plate to a high temperature in the Vosstian iron zone is continuously developing. Hot stamping is attracting attention as a technology that can take care of both forming and securing strength of automotive components because it is quenched in the mold while pressing.

On the other hand, a formed body obtained by forming a high-strength steel plate by hot embossing is required to have a performance capable of absorbing an impact during a collision.

As a technique that satisfies this requirement, Patent Document 1 discloses that annealing a steel sheet for hot stamping, and concentrating Mn or Cr in carbides to make hardly soluble carbides, thereby utilizing this during hot stamping heating. The technology of waiting for carbides to suppress the growth of Vostian iron to make it finer.

Patent Document 2 discloses a technique for finely granulating Vostian iron by increasing the temperature at a heating rate of 90 ° C./s or lower during hot stamping heating.

Patent Document 3, Patent Document 4, and Patent Document 5 also disclose techniques for fine-graining Vosstian iron to improve toughness.

Prior Art Literature Patent Literature Patent Literature 1: International Patent Publication No. 2015/147216 Patent Literature 2: Japanese Patent Laid-Open No. 5369714 Patent Literature 3: Japanese Patent Laid-Open No. 5114691 Patent Literature 4: Japanese Patent Laid-Open No. 2014-15638 Patent Publication No. 5: Japanese Patent Laid-Open No. 2002-309345

SUMMARY OF THE INVENTION Problems to be Solved by the Invention However, with the techniques disclosed in the aforementioned Patent Documents 1 to 5, it is difficult to obtain more fine-grained Vosstian iron, and it is impossible to expect to obtain higher strength or bending deformation ability than in the past.

The present invention has been made in view of the problems of the prior art, and has as its object to ensure more excellent strength or bending deformation ability in a hot-embossed formed body of a high-strength steel sheet, and an object thereof is to provide a steel sheet for hot-embossing that can solve the problem.

Means for Solving the Problems The present inventors have conducted intensive studies on a method for solving the above problems. As a result, it was found that by making the grain size of the old Vosted iron of the hot-embossed molded body to be 3 μm or less, it was possible to obtain more excellent strength than before.

In addition, it was found that the particle size of the old Vosted iron which was to be hot-embossed was 3 μm or less, as long as the number density of the citronite or ε carbide in the steel sheet before forming was 1 × 10 ¹⁶ pieces / cm ³ or more, and the one or two kinds of Nb and Mo solid solution of the old Worth Tian Tiejing boundaries so brittle grain boundary strength can be increased.

In addition, it is also found that the crystal orientation of the grains of the toughened iron, Asada iron, or tempered Asada iron, which are controlled by hot stamping, is {112} <111>, which is the X-ray random intensity ratio. Utilizing the texture memory effect of Vostian Iron and Asada Iron, it can generate a crystal orientation with a high crack suppression effect in the hot stamped formed body, and can obtain excellent bending deformation in the hot stamped formed body. ability.

The invention of this case was made based on the above-mentioned findings and further research. The gist of the invention is as follows.

(1) A steel sheet for hot stamping, characterized in that its component composition is contained in mass%: C: 0.35% or more and 0.75% or less, Si: 0.005% or more and 0.25% or less, and Mn: 0.5% or more 3.0% or less, sol.Al: 0.0002% or more and 3.0% or less, Cr: 0.05% or more and 1.00% or less, B: 0.0005% or more and 0.010% or less, Nb: 0.01% or more and 0.15% or less Below, Mo: 0.005% or more and 1.00% or less, Ti: 0% or more and 0.15% or less, Ni: 0 or more and 3.00% or less, P: 0.10% or less, S: 0.10% or less, N: 0.010% Below, and the remaining part is Fe and unavoidable impurities; and the microstructure contains at least one of the toughened iron, Asada loose iron and tempered Asada loose iron in an area ratio of more than 90%; the grain boundary solid solution ratio Z is above 0.4, and the aforementioned grain boundary solid solution ratio Z is defined as Z = (mass% of Nb and Mo in grain boundaries) / (one or two of Nb and Mo when dissolved) Mass%); The random X-ray intensity ratio of {112} <111> constituting the grains of the lower toughened iron, Asada iron, or tempered Asada iron is 2.8 or more; and the Xueming particle size is 50nm or less Carbon iron and ε The number density of the total amount of compound 1 × 10 ¹⁶ atoms / cm ³ or more.

(2) The steel plate for hot stamping as described in said (1) which has a plating layer.

Advantageous Effects of Invention According to the present invention, it is possible to provide a steel sheet for hot stamping which can be used as a blank for a hot stamped molded article having excellent strength and bending deformation ability.

Forms for Carrying Out the Invention The present invention is characterized in that the number density of citronite or ε carbide is 1 × 10 ¹⁶ pieces / cm ³ or more, and one or two kinds of Nb and Mo are dissolved in the old solution. In the Vostian iron grain boundary, the embrittlement strength of the grain boundary is increased. In addition, the crystal orientation of the grains of toughened iron, Asada scattered iron or tempered Asada scattered iron below the steel plate, that is, the random intensity ratio of X-rays of {112} <111>. As a result of intensive research, the present inventors have found that the above-mentioned structure can be obtained by the following method.

As the first stage, the amount of molten steel cast per unit time is controlled. Thereby, the micro-segregation of Mn in the steel sheet is suppressed, and the precipitation of Mo and Nb is further suppressed, so that the solid solution amount of Mo and Nb in the steel is increased.

If the amount of molten steel cast per unit time is controlled to reduce the microsegregation of Mn, the trap site of P will disappear, so the completion rolling delay P will segregate in the old Vostian iron grain boundary. In this way, although the grain boundaries of the old Vostian iron have been fine-grained, the embrittlement strength of the grain boundaries will be reduced, and the impact absorption capacity cannot be fully obtained. This is because the affinity of Mn and P is high, segregation of Mn will function as the trap position of P, and eliminating segregation will cause P to diffuse in the old Vostian iron grain boundary. In the present invention, this problem is solved by controlling the rolling conditions in the second stage.

As the second stage, the reduction of Mn into carbides is controlled by controlling the reduction rate, temperature, cooling conditions after rolling and the coiling temperature of hot-rolled rolling, so as to generate easily-dissolved fine carbides. , And then introduce high density differential rows into the steel. In the present invention, finely dispersed carbides and high-density differential rows become the inverted state of Vosstian iron to refine the old Vosstian iron particles. In order to effectively perform the function as an inversion state, the carbide should preferably be easily soluble. For this reason, it is important to prevent elements that inhibit the dissolution of carbides, such as Mn and Cr, from being concentrated in the carbides.

In addition, the precipitation of Mo and Nb is suppressed, and Nb and Mo are dissolved in the old Vostian iron grain boundary, thereby using Nb and Mo to occupy the segregation position of P, thereby eliminating the segregation of P to the old Vostian iron. Thus, not only can Mo or Nb be used to increase the grain boundary strength, but also it is possible to suppress a decrease in grain boundary embrittlement strength.

In addition, by controlling the coiling conditions, the concentration of Mn in the carbides is suppressed to generate easily-dissolved fine carbides, and by introducing high-density differential discharge into the steel, Vostian Iron can be made. When the strength of the iron increases, the crystalline orientation that is beneficial to alleviate the stress caused by the metamorphism will be preferentially generated when the iron is transformed from the wastfield iron to the down-toughened iron, the Asada iron, or the tempered Asada iron. As a result, the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled.

By controlling the heating speed of the hot stamping step, the steel plate for hot stamping exhibits different characteristics.

Hereinafter, the steel sheet for hot stamping of the present invention and a method for manufacturing the same will be described. First, the reason for limiting the component composition of the steel sheet for hot stamping of the present invention will be explained. Hereinafter, the symbol% regarding the component composition means mass%.

"C: 0.35% or more and 0.75% or less" In order to obtain a tensile strength of 2000 MPa or more for a hot stamped molded body, C is an important element. If it is less than 0.35%, Asada loose iron is relatively soft, and it is difficult to ensure a tensile strength of 2000 MPa or more. Therefore, the C system is set to 0.35% or more. It should be more than 0.37%. In view of the balance between the required strength and the suppression of early fracture, the upper limit is set to 0.75%.

"Si: 0.005% or more and 0.25% or less" Si is an element that improves the deformation ability and contributes to the improvement of the shock absorption ability. If it is less than 0.005%, the deformability is poor and the impact absorbing ability of the hot stamped molded body is deteriorated. Therefore, 0.005% or more is added. It should be more than 0.01%. On the other hand, if it is more than 0.25%, the amount of solid solution in the carbide increases, making the carbide difficult to dissolve, and it becomes impossible to control the old Vosted iron particle size of the hot-embossed molded body to 3 μm. Therefore, the upper limit is set to 0.25%. It should be 0.22% or less.

"Mn: 0.5% or more and 3.0% or less" Mn is an element that contributes to strength improvement by solid solution strengthening. If it is less than 0.5%, the solid solution strengthening ability is poor and Asada loose iron becomes soft, and the hot stamped molded body is difficult to ensure a tensile strength of 2000 MPa or more, so 0.5% or more is added. It should be more than 0.7%. On the other hand, if it is added more than 3.0%, the amount of solid solution in the carbide will increase, making the carbide difficult to dissolve, and it will be impossible to control the old Vosted iron particle size of the hot stamped compact to 3 μm. Below, the upper limit is 3.0%. It should be 2.5% or less.

"Sol.Al: 0.0002% or more and 3.0% or less" Al is an element capable of exerting a function of deoxidizing molten steel and improving the soundness of the steel. If it is less than 0.0002%, it will be sufficiently deoxidized and coarse oxides with a diameter of 5 μm or more will be generated, leading to early fracture. Therefore, the sol.Al system should be 0.0002% or more. It should be more than 0.0010%. On the other hand, if it is added more than 3.0%, coarse oxides are generated and the toughness is impaired, so it is set to 3.0% or less. It should be 2.5% or less, and more preferably 0.5% or less.

"Cr: 0.05% or more and 1.00% or less" Cr is an element that contributes to strength improvement by solid solution strengthening. If it is less than 0.05%, the lack of solid solution strengthening ability causes softening of Asada's loose iron, and it is difficult for the hot stamped molded body to ensure a tensile strength of 2000 MPa or more. Therefore, it is necessary to add 0.05% or more. And it should be above 0.1%. On the other hand, if it is added more than 1.00%, the solid solution amount in the carbide will increase, making the carbide difficult to dissolve, and it will be impossible to control the old Vosted iron particle size of the hot stamped molded body to 3 μm. Below, the upper limit is 1.00%. It should be 0.8% or less.

"B: 0.0005% or more and 0.010% or less" B is an element that contributes to strength improvement by solid solution strengthening. If it is less than 0.0005%, the solid solution strengthening ability is poor, and Asada scattered iron becomes soft, and the hot stamped molded body is difficult to ensure a tensile strength of 2000 MPa or more. Therefore, it is necessary to add 0.0005% or more. It should be more than 0.0008%. On the other hand, if it is added more than 0.010%, the amount of solid solution in the carbide will increase, making the carbide difficult to dissolve, and it will be impossible to control the old Vosted iron particle size of the hot stamped compact to 3 μm. Hereinafter, the upper limit is 0.010%. It should be 0.007% or less.

"Nb: 0.01% or more and 0.15% or less" Nb is an element that solid-dissolves in the old Vostian iron grain boundary and increases the strength of the grain boundary. In addition, Nb can be dissolved in the grain boundaries to prevent the grain boundary segregation of P, so the brittleness strength of the grain boundaries can be improved. In addition, by dissolving Nb and Mo in the Vosstian iron which has just finished rolling, and controlling the coiling conditions, the strength of Vosstian iron can be increased, and it will change from the transformation of Vosstian iron to downward. In the case of ductile iron, Asada loose iron, or tempered Asada loose iron, the crystalline orientation that is favorable for alleviating the stress caused by metamorphosis will be preferentially generated. As a result, the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled. Therefore, it is necessary to add more than 0.01%. It should be more than 0.030%. On the other hand, if it is added more than 0.15%, it will be easy to precipitate as a carbide, and the amount of solid solution at the grain boundary will be reduced. Therefore, it is set to 0.15% or less. And it should be below 0.12%.

"Mo: 0.005% or more and 1.00% or less" Mo is an element that solidly dissolves in old Vostian iron grain boundaries and increases grain boundary strength. In addition, since Mo is solid-dissolved in the grain boundaries, it can hinder the grain boundary segregation of P, so the brittleness strength of the grain boundaries can be improved. In addition, by dissolving Nb and Mo in the Vosstian iron which has just finished rolling, and controlling the coiling conditions, the strength of Vosstian iron can be increased, and it will change from the transformation of Vosstian iron to downward. In the case of ductile iron, Asada loose iron, or tempered Asada loose iron, it is preferentially formed although it is a crystalline orientation that is good for alleviating the stress caused by metamorphosis. As a result, the X-ray random intensity ratio of {112} <111> of the crystal grains can be controlled. Therefore, it is necessary to add more than 0.005%. It should be more than 0.030%. On the other hand, if it is added more than 1.00%, it will become easy to precipitate as a carbide, resulting in a decrease in the amount of solid solution at the grain boundaries. Therefore, it is set to 1.00% or less. It should be below 0.80%.

"Ti: 0% or more and 0.15% or less" Although Ti is not an essential element, Ti is an element that contributes to strength improvement by solid solution strengthening, so it can be added as needed. When Ti is added, it should be set to 0.01% or more in order to obtain the addition effect. It should be 0.02%. On the other hand, if it is added more than 0.15%, coarse carbides or nitrides having a diameter of 5 μm or more are formed and early fracture is caused. Therefore, it is set to 0.15% or less. And it should be 0.12% or less.

"Ni: 0% or more and 3.00% or less" Although Ni is not an essential element, it is an element that contributes to strength improvement by solid solution strengthening, so it can be added as needed. When Ni is added, it should be set to 0.01% or more in order to obtain the addition effect. It should be 0.02%. On the other hand, if it is added more than 3.00%, the steel will become brittle and cause early fracture, so it is set to 3.00% or less. It should be less than 2.00%.

"P: 0.10% or less" P is an element that is an impurity and is an element that is liable to segregate in grain boundaries and reduces the embrittlement strength of the grain boundaries. If it is more than 0.10%, the embrittlement strength of the grain boundary will be significantly reduced and early fracture will be caused. Therefore, the P system is set to 0.10% or less. And it should be 0.050% or less. Although the lower limit is not particularly limited, if it is reduced to less than 0.0001%, the cost of P removal will increase sharply and become unfavorable to the economy. Therefore, in practical steel plates, 0.0001% is the actual lower limit.

"S: 0.10% or less" S is an impurity element and is an element that forms inclusions. If it is more than 0.10%, inclusions may be generated and early fracture may be caused. Therefore, the S content is set to 0.10% or less. It should be 0.0050% or less. Although the lower limit is not particularly limited, if it is reduced to less than 0.0015%, the cost of de-S will increase sharply and become unfavorable to the economic side. Therefore, on a practical steel plate, 0.0015% is the actual lower limit.

"N: 0.010% or less" Since N is an impurity element and nitrides are formed to cause early fracture, it is set to 0.010% or less. And it should be 0.0075% or less. Although the lower limit is not particularly limited, if it is reduced to less than 0.0001%, the cost of denitrification will increase sharply and become unfavorable to the economy. Therefore, on a practical steel plate, 0.0001% is the actual lower limit.

The remainder of the composition is Fe and impurities. Examples of the impurities include elements that are unavoidably mixed from steel raw materials or scrap materials and / or during the steel making process, and that are allowed within a range that does not impede the characteristics of the hot stamped molded body of the present invention.

Next, the reason for limiting the microstructure of the hot stamping steel sheet of the present invention will be described.

"In terms of microstructure area ratio, more than 90% is one of the lower toughened iron, Asada loose iron and tempered Asada loose iron."

In order to obtain a tensile strength of 1500 MPa or more for the hot-embossed formed body, the microstructure of the steel sheet for hot-embossing must contain 90% or more of Asada loose iron or tempered Asada loose iron in terms of area ratio. It should be above 94%. From the viewpoint of ensuring tensile strength, the microstructure may also be lower toughened iron. The remaining portion is not particularly specified, and examples thereof include upper toughened iron, residual Vosda iron, and Pola iron.

The area ratios of the lower toughened iron, Asada loose iron, and tempered Asada loose iron were measured as follows.

Cut out a cross section perpendicular to the plate surface from the center of the hot stamping steel sheet, grind the measurement surface with # 600 to # 1500 silicon carbide paper, and use diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or The liquid dispersed in pure water is processed into a mirror surface.

Immerse in a 1.5 ~ 3% nitric acid-alcohol solution for 5 ~ 10 seconds to expose the high-angle grain boundaries. At this time, the corrosion operation is performed in the exhaust treatment device, and the temperature of the working gas environment is set to normal temperature.

The etched sample was washed with acetone or ethanol, dried, and then subjected to scanning electron microscope observation. The scanning electron microscope used was a thing equipped with a 2-electron detector. In a vacuum of 9.6 × 10 ^-5 or less, the sample is irradiated with electron beams at an acceleration voltage of 10 kV and an irradiation current level of 8 and the range of 1/8 to 3/8 positions is photographed around the 1/4 plate thickness position of the sample. Secondary electronic image. The shooting magnification is set to 10,000 times based on a screen of 386 mm x 290 mm in length, and the number of shooting fields of view is set to 10 or more.

In the photographed secondary electron image, since the crystal grain boundaries and the carbide system are photographed as a bright contrast, the structure can be easily determined by the positions of the crystal grain boundaries and the carbides. When carbides are formed inside the grains, they are tempered Asada loose iron or lower toughened iron, and the structure in the grains where no carbides are observed inside is Asada loose iron.

On the other hand, in a structure system in which carbides are formed in crystal grain boundaries, iron or boron is toughened.

Regarding the residual Vostian iron, since the crystal structure is different from the above-mentioned microstructure, it is measured by the electron backscatter diffraction method in the same field of view as the position where the secondary electron image is taken. The scanning electron microscope used was a thing equipped with a camera capable of performing an electron backscatter diffraction method. In a vacuum of 9.6 × 10 ^-5 or less, the sample was irradiated with an electron beam at an acceleration voltage of 25 kV and an irradiation current level of 16 for measurement, and a distribution map of the face-centered cubic lattice was prepared based on the obtained measurement data.

The shooting magnification is based on a screen of 386 mm x 290 mm in length and 10,000 times. Then, a 2 μm grid is formed on the photograph obtained, and the microstructure at the intersection of the grids is selected. The area fraction of the microstructure is the value obtained by dividing the number of intersections of each tissue by all the intersections. This operation was performed in 10 fields of view, and the average value was calculated and used as the area ratio of the microstructure.

"The grain boundary solid solution ratio Z defined by formula (1) is above 0.4"

Z = mass% of one or two kinds of Nb and Mo in grain boundaries / mass% of one or two kinds of Nb and Mo when dissolved ... (1)

The grain boundary solid solution ratio Z defined by the above formula (1) is an important tissue factor in terms of ensuring excellent impact absorption capacity, and is an index adopted by the inventors for evaluating the impact absorption capacity. If Nb and / or Mo are solid-dissolved in the grain boundaries, P will become difficult to segregate in the grain boundaries and the cohesion of the grain boundaries will be improved. Therefore, the embrittlement strength of the grain boundaries will increase and the impact absorption capacity will be improved. If the grain boundary solid solution ratio Z of the hot-embossed molded body is less than 0.4, the grain boundary strengthening effect of Nb and / or Mo cannot be sufficiently obtained, and the required impact absorption capability cannot be obtained. When the steel sheet for hot stamping is supplied for hot stamping, the grain boundary solid solution amount of Nb and Mo is reduced by heat treatment, so the grain boundary solid solution ratio Z is set to 0.4 or more. It should preferably be 0.5 or more. Although the upper limit is not particularly limited, theoretically 1.0 is the upper limit.

The grain boundary solid solution ratio Z is measured as follows.

A test piece having a size shown in FIG. 1 was produced from the center of the steel sheet for hot stamping. At this time, the front and back surfaces of the test piece were removed by the same amount each time by mechanical grinding to make the plate thickness 1.2 mm. The incision at the center of the test piece was inserted with a 1 mm thick wire cutter, and the joint at the bottom of the incision was controlled to 100 μ to 200 μm.

Next, the test piece was immersed in a 20% -ammonium thiocyanate solution for 72 to 120 hours.

The surface of the test piece was galvanized within 0.5 hr after the immersion was completed.

After plating, it will be used for analysis of Auje electronic luminescence within 1.5hr. There are no particular restrictions on the type of device used to perform the Auger electron emission spectroscopic analysis. The test piece was set in an analysis device, and the test piece was broken from a notch portion of the test piece in a vacuum of 9.6 × 10 ^-5 or less to expose the old Vostian iron grain boundary. Electron rays were irradiated to the exposed old Vostian iron grain boundaries at an acceleration voltage of 1 to 30 kV, and the mass% (concentration) of Nb and / or Mo in the grain boundaries was measured. The measurement was performed at 10 or more old Vostian iron grain boundaries. In order to prevent grain boundary contamination, the measurement will be completed within 30 minutes after the destruction.

After calculating the average value of the measured mass% (concentration) of Nb and / or Mo, the value obtained by dividing the average value by the added mass% of Nb and / or Mo is taken as the grain boundary solid solution ratio Z. .

"The random X-ray intensity ratio of {112} <111> constituting the grains of the lower toughened iron, Asada iron, or tempered Asada iron is 2.8 or more"

In the steel plate for hot stamping, if the {112} <111> X-ray random intensity ratio of the grains of the lower toughened iron, Asada iron, or tempered Asada iron is less than 2.8, it will be used in the hot stamped formed body. A crystal orientation with a high effect of suppressing crack propagation is not generated, and 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. Although there is no specific upper limit, it is difficult to make it more than 15.0 in actual operation, so 15.0 is a substantial limit.

Next, a calculation method of the metal structure will be described.

A sample was cut out from the center portion of the steel sheet for hot stamping so that a cross section (plate thickness cross section) perpendicular to its surface could be observed. Polish the measurement surface with # 600 to # 1500 silicon carbide paper, and use a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water to process a mirror surface.

Next, finish grinding was performed using a standard colloidal silica suspension (particle diameter: 0.04 μm). The ground sample was washed with acetone or ethanol, dried, and set in a scanning electron microscope. The scanning electron microscope used was one equipped with an EBSD detector (DVC5 type detector made by TSL).

In the 3 / 8th to 5 / 8th position of the plate thickness of the sample, EBSD measurement was performed on a range of 500 μm in the thickness direction and 1000 μm in the rolling direction at a measurement interval of 0.2 μm to obtain crystal orientation information. The measurement conditions were set to a vacuum degree 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.

The measurement software was analyzed using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, and the X-ray random intensity ratio of {112} <111> was calculated. Use the "Texture" function and the "Crystal orientation distribution function" function installed in the software to draw the crystal orientation distribution function with a section of φ ₂ = 45 °. Read the X-ray random intensity ratio of the end position of {112} <111> from the drawn image.

"Total number density of citronite and ε carbides with a particle size below 50nm is 1 × 10 ¹⁶ pieces / cm ³ or more" As long as the number density of citronite and ε carbides with a particle size below 50nm When the total is 1 × 10 ¹⁶ pieces / cm ³ or more, the finely dispersed carbides will become the inverse position of Vosstian iron, thereby miniaturizing the old Vosstian iron grains of the hot-embossed formed body. Since 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 . And it is preferably 3 × 10 ¹⁶ pieces / cm ³ . Although there is no particular upper limit, the upper limit is set to 1000 × 10 ¹⁶ pieces / cm ^{3 in} view of the balance between the required strength and the suppression of early fracture. In addition, as long as the steel sheet is manufactured under the manufacturing conditions specified in this case, the carbides produced will mainly be citronite and ε carbides.

Next, a calculation method of the metal structure will be described.

From the steel plate for hot stamping, a sample was cut out so that a cross section (plate thickness cross section) perpendicular to the surface could be observed. After polishing the measurement surface with # 600 to # 1500 silicon carbide paper, a liquid obtained by dispersing diamond powder with a particle size of 1 μm to 6 μm in a diluent such as alcohol or pure water is processed into a mirror surface.

Next, electric field etching was performed using the SPEED method using a non-water-soluble electrolyte as described in "Fumio Kurosawa, Taguchi Yu, Ryuta Matsumoto, Journal of the Japanese Metal Society, 43, 1068 (1979)", and the sample was adjusted to allow easy observation of fine carbonization. Thing. This method uses different decomposition potentials of carbon steel and cis-carbon iron or ε carbide, and by performing electrolysis with a potential that only base iron can decompose, it is possible to easily observe carbides. By using a water-insoluble electrolytic solution, decomposition of water-soluble cuming iron or ε carbide can be suppressed, and therefore it is suitable for measuring the size of fine carbides or measuring the number density.

The observation surface of the sample was immersed in an acetoacetone-based electrolytic solution, and electrolysis was performed at an electrolysis potential of 300 mV for 2 seconds. The electrolyzed sample was washed with acetone or ethanol, dried, and set in a scanning electron microscope. The scanning electron microscope used was a model equipped with a secondary electron detector. In a vacuum of 9.6 × 10 ^-5 or less, the sample was irradiated with electron rays at an acceleration voltage of 10 kV and an irradiation current level of 8, and the sample thickness was from 3/8 to 5/8, with a width of 386 mm and a length of 290 mm. This screen is used as a reference, and 10 fields of view with a magnification of 30,000 times are observed.

The number of citronite and ε carbides having a particle diameter (major axis length) of 50 nm or less in the observation field was measured. Calculate the value obtained by dividing the number of the above-mentioned carbides contained in one visual field by the area of the observation visual field. The same operation was performed in 10 fields of view, and the average value of all fields of view was used as the number density of citronite and ε carbide.

Next, the form of the manufacturing method for obtaining the hot stamping steel sheet of this invention is demonstrated.

<Manufacturing method of steel plate for hot stamping>

(1) Continuous casting step A molten steel having the above-mentioned chemical composition is made into a steel sheet (steel blank) by a continuous casting method. In this continuous casting step, the amount of molten steel cast per unit time is set to 6 ton / minute or less. In continuous casting, if the casting amount per unit time (casting speed) of the molten steel is greater than 6 ton / minute, the microsegregation of Mn increases, and the nucleation amount of precipitates mainly composed of Mo or Nb will increase. The amount of casting is more preferably set to 5 ton / minute or less. The lower limit of the amount of casting is not particularly limited, but from the viewpoint of operating cost, it is preferably 0.1 ton / min or more.

(2) Hot rolling step The above-mentioned steel sheet is hot rolled to form a steel sheet. At this time, hot rolling is terminated in a temperature range defined by the formula (2) A3 abnormal temperature + 30 ° C or higher and A3 abnormal temperature + 200 ° C or lower, and the final stage at this time has a rolling reduction rate of 12% or more. , Cooling starts within 1 second after the completion of rolling, and in the temperature range from the completion of rolling to 550 ° C, at a cooling rate of 100 ° C / sec or more, and at a temperature of less than 500 ° C Take up.

A3 abnormal temperature = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo .... Formula (2)

By setting the finish rolling temperature to A3 transformation temperature + 30 ° C or higher, the recrystallization of Vostian iron is promoted. Thereby, the formation of low-angle grain boundaries in the crystal grains is suppressed, and the precipitation positions of Nb and Mo can be reduced. The A3 transformation temperature is preferably + 50 ° C or more.

By setting the finish rolling temperature to A3 transformation temperature + 200 ° C or lower, excessive grain growth of Vosstian iron is suppressed. By finishing rolling in a temperature range of A3 abnormal temperature + 200 ° C or less, the recrystallization of Vostian iron can be promoted without excessive grain growth, so fineness can be obtained in the coiling step. carbide. The A3 transformation temperature is preferably 150 ° C or lower.

By setting the rolling reduction of the finished rolling to 12% or more, the recrystallization of Vosted iron is promoted. Thereby, the formation of low-angle grain boundaries in the crystal grains is suppressed, and the precipitation positions of Nb and Mo can be reduced. It is preferably at least 15%.

The cooling is started within 1 second and preferably within 0.8 seconds after the completion of rolling, and the cooling is performed at a cooling rate of 100 ° C / second or higher in a temperature range from the completion of rolling to the temperature of 550 ° C. This can reduce the residence time in a temperature region that promotes the precipitation of Nb and Mn. As a result, the precipitation of Nb and Mo in the Vosstian iron can be suppressed, and the solid solution amounts of Nb and Mo in the Vosstian iron grain boundary increase.

By setting the coiling temperature to less than 500 ° C, the above-mentioned effects are enhanced, and at the same time, Mn is inhibited from being concentrated in carbides, and fine carbides that are easily dissolved are generated, and high density differential discharge is introduced into the steel. It is preferably lower than 480 ° C. If the coiling temperature is higher than 500 ° C., the total number density of citronite and ε carbides having a particle diameter of 50 nm or less will not be 1 × 10 ¹⁶ pieces / cm ³ or more. Although there is no special lower limit, it is difficult to take up coils at room temperature or lower in actual operation, so room temperature is the lower limit.

In addition, immediately after completion of rolling, Nb and Mo will be dissolved in Vosstian iron. By transforming Vostian iron with Nb and Mo in solution, it will be transformed into lower-toughened iron, Asada loose iron, or tempered Asada loose iron. Iron, Nb, and Mo preferentially generate crystal orientations that are beneficial to alleviate the stress caused by metamorphosis. Therefore, as mentioned above, the cooling is started within 1 second after the completion of rolling and the temperature at the end of rolling is completed. Cooling at a cooling rate of 100 ° C / sec or higher in a temperature range from 550 ° C to above can control the X-ray random intensity ratio of {112} <111> of the crystal grains.

(3) Formation of a plating layer A plating layer can also be formed on the surface of a steel sheet for the purpose of improving corrosion resistance and the like. The plating layer may be any of a plating layer and a molten plating layer. Examples of the electroplated layer include an electroplated zinc layer and an electroplated Zn-Ni alloy layer. Examples of the hot-dip coating include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum coating, a hot-dip Zn-Al alloy coating, a hot-dip Zn-Al-Mg alloy coating, and a hot-dip Zn-Al-Mg-Si alloy. Plating, etc. The amount of the plating layer is not particularly limited, and may be a general amount of adhesion.

(4) Other steps In the production of the hot stamping steel sheet, other well-known manufacturing methods such as pickling, cold rolling, and temper rolling can also be included.

<An example of the manufacturing process of a hot stamping molded body> Next, the form of the manufacturing method for manufacturing a hot stamping molded body using the hot stamping steel plate of this invention is demonstrated. The method for producing a hot-embossed molded body is not limited to the following forms.

(Manufacturing method A) A method for producing a hot-stamped molded article having excellent strength is a steel plate for hot-stamping in a temperature range of an average heating rate of 100 ° C / s or more and less than 200 ° C / s in a temperature range of 500 ° C or more and A3 point or less. After heating and maintaining, hot-embossing is performed, and the formed body is cooled to room temperature after the forming. In addition, in order to adjust the strength, a part or all of the area of the hot stamped molded body may be tempered at a temperature of 200 ° C or higher and 500 ° C or lower.

By heating at an average heating rate of 100 ° C / s or more and less than 200 ° C / s in a temperature range of 500 ° C or more and A3 point or less, it is possible to dissolve both easily-dissolved fine carbides and high-density differential rows The nucleation site of the old Vosstian iron is made, and the average particle size of the old Vosstian iron can be controlled to be less than 3 μm. In addition, it also helps to suppress the precipitation of NbC and MoC during heating, and increases the solid solution ratio of one or two of Nb and Mo in the old Vostian iron grain boundaries. It is preferably 120 ° C / s or more. If the average heating rate is higher than 200 ° C / s, the transformation to Vostian iron is promoted in the state where the dissolution of carbides is not completed, and the toughness is deteriorated. Therefore, the upper limit is 200 ° C / s. And it should be less than 180 ℃ / s.

The maintenance temperature during hot embossing should be set to A3 point + 50 ° C or more and A3 point + 150 ° C or less. In addition, the cooling rate after hot stamping should be set to 10 ° C / s or more.

(Manufacturing method B: Method for manufacturing hot-embossed molded articles with excellent bending deformation) A steel sheet for hot-stamping, a steel sheet obtained by cold-rolling the steel sheet, or a steel plate obtained by plating the steel sheet After heating at an average heating rate of less than 100 ° C / s to a point A3 or more and maintaining it, hot stamping is performed, and the formed body is cooled to room temperature after the forming. In addition, in order to adjust the strength, a part or all of the area of the hot stamped molded body may be tempered at a temperature of 200 ° C or higher and 500 ° C or lower.

The maintenance temperature during hot embossing should be set to A3 point + 10 ° C or more and A3 point + 150 ° C or less. In addition, the cooling rate after hot stamping should be set to 10 ° C / s or more. Examples

Next, the embodiments of the present invention will be described. However, the conditions in the embodiments are only one example of conditions used to confirm the feasibility and effect of the present invention, and the present invention is not limited by the one example of conditions. As long as the purpose of the present invention can be achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

The steel sheet obtained by casting molten steel having the composition shown in Table 1 was subjected to hot rolling as shown in Table 2 to produce a steel sheet for hot stamping. For the prepared hot stamping steel sheet, the area ratios of the lower toughened iron, Asada loose iron and tempered Asada loose iron, the grain boundary solid solution ratio of Nb and Mo, and the composition of the lower toughened steel were measured by the methods described above. The X-ray random intensity ratio of {112} <111> of the grains of iron, Asada scattered iron, or tempered Asada scattered iron, and the number density of cis-carbon iron and ε carbide with a particle diameter below 50nm.

In addition, using the obtained hot stamping steel sheet, cold rolling and plating were performed under the conditions shown in Table 3 to prepare a hot stamped molded body. The heat treatment at the time of hot embossing is performed at various speeds in an average heating rate in a temperature range of 500 ° C or higher and A3 or lower.

[Table 1-1]

[Table 1-2]

[Table 1-3]

[table 2-1]

[Table 2-2]

[Table 2-3]

[Table 3-1]

[Table 3-2]

[Table 3-3]

A sample made of a hot stamped molded body with an average heating rate in a temperature range of 500 ° C or higher and A3 or lower to 100 ° C / s or higher was measured for tensile strength and evaluated for impact absorption.

A sample made of a hot-embossed molded body with an average heating rate in a temperature range of 500 ° C or higher and A3 or lower was set to less than 100 ° C / s, the tensile strength was measured, and the bending deformation ability was evaluated.

In addition, the impact absorption capacity was evaluated based on the presence or absence of early fracture, and a material that did not undergo early fracture in the evaluation criteria described below was qualified. The term "excellent shock absorption capability" means that the energy absorption amount during collision is large. That is, the integral value of the stress-strain curve is large, and it can be evaluated based on the condition that no early fracture occurs (fracture occurs after reaching the maximum stress).

The value obtained by dividing the maximum strength obtained in the tensile test by 3.3 times the Vickers hardness of the material, and when the obtained value is 0.85 or more, it is judged that early fracture has been suppressed. The Vickers hardness of the material was measured by the following method.

Cut out a cross section perpendicular to the board surface from the hot stamped molded body, grind the measurement surface with # 600 to # 1500 silicon carbide paper, and use diamond powder with a particle size of 1 to 6 μm in diluent such as alcohol or pure water. The dispersed liquid is processed into a mirror surface. Using a Vickers hardness tester, 10 points were measured with a load of 1 kgf at a 1/4 position of the plate thickness and a measurement interval of 3 times or more the indentation, and the average value was used as the hardness of the steel plate.

The bending deformation ability was evaluated according to the VDA standard (VDA238-100) stipulated by the German Automobile Industry Association under the following measurement conditions. In the present invention, the displacement at the maximum load obtained in the bending test is converted into an angle on the basis of the VDA, and the maximum bending angle is obtained. A material having a maximum bending angle of 50 ° or more is acceptable.

Test piece size: 60mm (rolling direction) × 30mm (direction perpendicular to rolling), plate thickness 1.0mm Bending ridge: direction perpendicular to rolling test method: roll support, punch extrusion roll diameter: φ30mm Punch shape: front end R = 0.4mm distance between rollers: 2.0 × 1.0 (mm) + 0.5mm extrusion speed: 20mm / min testing machine: SHIMADZU AUTOGRAPH 20kN

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 deformation ability. On the other hand, in the case where the chemical composition and the manufacturing method are inappropriate, the target characteristics cannot be obtained.

FIG. 1 is a view showing the shape of a test piece when the grain boundary solid solution ratio is measured.

Claims (2)

A steel sheet for hot stamping, characterized in that: its component composition is contained in mass%: C: 0.35% or more and 0.75% or less, Si: 0.005% or more and 0.25% or less, Mn: 0.5% or more and 3.0 % Or less, sol.Al: 0.0002% or more and 3.0% or less, Cr: 0.05% or more and 1.00% or less, B: 0.0005% or more and 0.010% or less, Nb: 0.01% or more and 0.15% or less, Mo : 0.005% or more and 1.00% or less, Ti: 0% or more and 0.15% or less, Ni: 0 or more and 3.00% or less, P: 0.10% or less, S: 0.10% or less, N: 0.010% or less, and The remaining part is Fe and unavoidable impurities; and the microstructure includes at least one of the toughened iron, Asa loose iron and tempered Asa loose iron in an area ratio of more than 90%; the grain boundary solid solution ratio Z is 0.4. As mentioned above, the aforementioned grain boundary solid solution ratio Z is defined as Z = (mass% of one or two of Nb and Mo in the grain boundary) / (mass% of one or two of Nb and Mo when dissolved) The random X-ray intensity ratio of {112} <111> constituting the grains of the lower toughened iron, Asada iron, or tempered Asada iron is 2.8 or more; and Xueming has a particle size of 50 nm or less The number density of iron and total carbide ε 1 × 10 ¹⁶ atoms / cm ³ or more. The steel sheet for hot stamping as claimed in claim 1, which has a plating layer.