TW201942391A - Hot-stamped formed product especially used for structural member or reinforcement member - Google Patents

Abstract

This hot-stamped formed product having excellent strength is characterized by having a prescribed component composition, and also characterized in that: the prior austenite having an average grain size of 3 [mu]m or less; containing at least one of the lower bainite, martensite, and tempered martensite, which are included at an area ratio of 90% or more; and the grain boundary solid/solution ratio Z, defined as Z = (mass% of at least one or two of Nb and Mo at the grain boundary)/(mass% of at least one or two of Nb and Mo at dissolution), is 0.3 or higher.

Description

Hot stamping

The present invention relates to a hot-embossed molded body having particularly excellent strength when used on a structural member or a reinforcing member of an automobile or a structure requiring strength.

BACKGROUND ART In recent years, from the viewpoint 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, for a molded body obtained by forming a high-strength steel plate by hot stamping, the performance of suppressing deformation (collision deformation suppression portion) during impact is necessary. Therefore, after hot stamping, it is necessary to Has high strength.

As a technology that can meet this requirement, Patent Document 1 discloses the following technique: annealing a steel sheet for hot stamping, concentrating Mn or Cr in carbides, and making hardly soluble carbides, thereby performing hot stamping During the heating, these carbides are used to suppress the growth of Vosted 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 produce a more fine-grained Vostian iron, and it is impossible to expect to obtain the above-mentioned strength.

The present invention has been made in view of the problems of conventional technologies, and has as its object to ensure more excellent strength in a hot-embossed molded body, and an object thereof is to provide a hot-embossed molded body capable of solving 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 the embrittlement strength of the grain boundaries was increased by reducing the particle size of the old Vostian iron to 3 μm or less and dissolving one or two kinds of Nb and Mo in the grain boundaries of the old Vostian iron. A better shock absorbing ability than before can be obtained.

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

A hot-embossed molded body, 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 more % 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 contains old Vostian iron with an average crystal grain size of 3 μm or less, and in area ratio, it contains more than 90% of toughened iron, Asada loose iron and recycled iron. At least one kind of loose iron in hemp field, and the grain boundary solid solution ratio Z is above 0.3. The aforementioned grain boundary solid solution ratio Z is defined as Z = (mass% of Nb and Mo in grain boundaries) / (Mass% of one or two of Nb and Mo when dissolved).

The hot stamped molded body according to the above (1), which has a plated layer.

Advantageous Effects of Invention According to the present invention, a hot-embossed molded body having excellent strength can be provided.

Embodiments of the invention The present invention is characterized in that the particle size of the old Vosted iron is 3 μm or less, and one or two kinds of Nb and Mo are dissolved in the grain boundary of the old Vosted iron to increase the embrittlement strength of the grain boundary. . 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.

As the third stage, by controlling the temperature rise rate during the hot stamping heating, both the easily dissolved fine carbides and the high-density differential rows become the nucleation sites of the old Vostian iron. This makes it possible to control the average particle size of the old Vosted iron in the hot-embossed molded body to 3 μm or less.

In addition, the precipitation of NbC and MoC during heating is suppressed so that the solid solution ratio of one or two of Nb and Mo in the old Vostian iron grain boundary is increased. In order to suppress the precipitation of Mo and Nb, it is necessary to make the temperature increase rate at the time of the hot stamping heating at least 100 ° C / s or more.

Hereinafter, the hot-embossed molded article of the present invention and a method for manufacturing the same will be described.

First, the reasons for limiting the composition of the components constituting the hot stamped molded body of the present invention will be described. Hereinafter, the% related to the component composition means the mass%.

"C: 0.35% or more and 0.75% or less" C is an important element for obtaining a tensile strength of 2000 MPa or more. 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 required strength and the balance to suppress 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 absorption capacity is deteriorated, so 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 particle size of the old Vostian iron to 3 μm, so 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 lack of solid solution strengthening ability will cause Asada loose iron to become soft, and it is difficult to ensure a tensile strength of 2000 MPa or more. Therefore, 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 particle size of the old Vostian iron to less than 3 μm, so 3.0% Is the upper limit. 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 to form coarse oxides and cause early fracture. Therefore, the sol.Al system is set to 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 early fracture is caused, so it is set to 3.0% or less. It should be 2.5% 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 will cause Asada scattered iron to soften, and it is difficult to ensure a tensile strength of 2000 MPa or more, so 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 amount of solid solution in the carbide will increase, making the carbide difficult to dissolve, and it will be impossible to control the particle size of the old Vostian iron to be less than 3 μm, so it is 1.00%. Is the upper limit. 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 lack of solid solution strengthening ability will cause Asada loose iron to become soft, and it 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 particle size of the old Vostian iron to less than 3 μm. Is the upper limit. 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. 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 0.12% or less.

"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. Therefore, it is necessary to add 0.005 or more. 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 are generated and early fracture is caused, so 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 reasons for limiting the microstructure of the hot-embossed molded article of the present invention will be described.

"The average grain size of the old Vostian iron is 3.0 μm or less"

The particle size of old Vosted iron is an important tissue factor to ensure excellent strength and the effect of suppressing early fracture. According to the study by the present inventors, in order to obtain the impact absorption capability required for the hot stamped formed body, the smaller the particle size of the old Vosted iron, the better, and the average particle size must be controlled to 3.0 μm or less. It is more preferably less than 2.7 μm, and the lower limit is not particularly limited. In the current actual operation, it is difficult to make it smaller than 0.5 μm, so 0.5 μm is the substantial lower limit.

The particle size of the old Vosted iron was measured as follows.

First, the hot-embossed molded body was heat-treated at 540 ° C for 24 hours. This promotes the corrosion of the old Vostian iron grain boundaries. The heat treatment may be performed by furnace heating or electric heating. The temperature increase rate is set to 0.1 to 100 ° C / s, and the cooling rate is set to 0.1 to 150 ° C / s.

A cross section perpendicular to the plate surface was cut out from the central portion of the hot-embossed molded body after the heat treatment, and the measurement surface was polished with # 600 to # 1500 silicon carbide paper, and then diamond powder with a particle size of 1 to 6 μm was used in alcohol, etc. Diluted liquid or liquid dispersed in pure water to be processed into a mirror surface.

Next, the observation surface was immersed in a 3 to 4% sulfuric acid-alcohol (or water) solution for 1 minute to expose the old Vostian iron 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 observation with a scanning electron microscope. The scanning electron microscope used was a thing equipped with a 2-electron detector.

In a vacuum below 9.6 × 10 ^-5 , the sample is irradiated with electron beams at an acceleration voltage of 15 kV and an irradiation current level of 13 and the position of the sample at the 1/4 plate thickness is taken as the center of the 1/8 to 3/8 position. Range of secondary electron images. The shooting magnification is set to 4000 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 fields of view.

In the captured secondary electron image, the old Vostian iron grain boundary system was photographed as a bright contrast. The average diameter of the shortest diameter and the longest diameter was calculated from the old Vostian iron particles contained in the observation field, and the particle diameter was set. The above operation is performed on all old Vosted iron particles except the old Vosted iron particles, such as the end of the shooting visual field, and the entire crystal grains are not included in the shooting visual field to obtain the average particle size in the shooting visual field. . The average particle diameter is a value obtained by dividing the total of the calculated particle diameters by the total number of old Vosted iron particles whose particle diameters have been measured. This operation was performed on all the fields of view obtained for each shot to calculate the average particle size of the old Vosstian iron.

"The grain boundary solid solution ratio Z defined by formula (1) is above 0.3"

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 is less than 0.3, the grain boundary strengthening effect of Nb and / or Mo cannot be sufficiently obtained, and the required impact absorption capacity cannot be obtained. Therefore, the grain boundary solid solution ratio Z is set to 0.3 or more. It should preferably be 0.4 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 central portion of the heat-embossed molded body after the heat treatment. 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. .

"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 molded body, the microstructure 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 structure with an area ratio of 90% or more may be one of lower toughened iron, Asada loose iron, and tempered Asada loose iron, or a mixed structure of these.

The remaining part of the microstructure is not particularly specified, and examples thereof include toughened iron, residual vostian iron, and pole 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 board surface from the center of the hot stamped molded body, grind the measurement surface with # 600 to # 1500 silicon carbide paper, and then use diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure The liquid dispersed in 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 observation with a scanning electron microscope. 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 rays at an acceleration voltage of 10 kV and an irradiation current level of 8; Range of secondary electron images. 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 electron rays 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.

"Manufacturing method of steel sheet for hot stamping" Next, a description will be given of the form of the hot stamped molded article of the present invention and a method for manufacturing a hot stamped steel sheet for manufacturing a hot stamped molded article. The invention is not limited to the form described below.

<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, it is desirable to set the molten steel casting amount per unit time to 6 tons / 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, the hot rolling is terminated in a temperature range defined by the formula (2) A3 abnormal temperature + 10 ° C or higher and A3 abnormal temperature + 200 ° C or lower, and the rolling reduction rate of the last stage at this time is set to 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 + 10 ° C or more, 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. In addition, since the consumption of C can also be suppressed by reducing the precipitation positions of Nb and Mo, the number density of carbides can be increased in the subsequent steps. The A3 transformation temperature is preferably + 30 ° 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 fine carbonization can be obtained in the coiling step. Thing. 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 from the completion of the rolling. The cooling is performed at a cooling rate of 100 ° C / second or higher in a temperature range from the completion of the 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 less than 480 ° C. 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.

(3) The plating layer is formed on the surface of the softened layer, and the plating layer can also be formed 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.

<Manufacturing steps of a hot stamped molded body>

The hot-embossed molded body of the present invention is heated and maintained by heating the steel sheet for hot-embossing in a temperature region with 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 that, hot embossing is performed, and after the forming, the formed body is cooled to room temperature to be manufactured.

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.

In a temperature range of 500 ° C or higher and A3 point or lower, heating and maintaining at an average heating rate of 100 ° C / s or more and less than 200 ° C / s, and performing hot embossing, thereby making it easy to dissolve fine carbides. Both the high-density differential rows are the nucleation sites of the old Vosstian iron, and the average particle size of the old Vosstian iron can be controlled to 3 μm or less. 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.

The average heating rate should be above 120 ° C / s. 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 + 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 examples of the present invention will be described. However, the conditions in the examples 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.

Regarding the steel sheet obtained by casting molten steel with the composition shown in Tables 1-1 to 1-3, hot rolling and cold rolling are performed as shown in Tables 2-1 to 2-3 to form a steel sheet for hot stamping. Then, the heat-embossed steel sheet was heat-treated as shown in Table 2 and hot-embossed to form a molded body.

Tables 3-1 to 3-3 show the evaluation results of the microstructure and strength of the hot-embossed molded body and the impact absorption capacity.

[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]

With respect to the hot-embossed molded body, the area ratios of the lower toughened iron, Asada iron, and tempered Asada iron were measured by the aforementioned method, and the grain boundary solid solution ratio of Nb and Mo was measured.

The strength of the hot stamped molded body was evaluated by a tensile test. The tensile test was carried out in accordance with the test method described in JIS Z 2241, and the test piece No. 5 described in JIS Z 2201 was produced, and the maximum strength was 2,000 MPa or more.

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 sheet.

The tensile strength of the hot-embossed molded article of the present invention is 2000 MPa or more, and it can be confirmed that early fracture has been suppressed. 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 hot-embossed molded body, 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 contains old Vostian iron with an average crystal grain size of 3 μm or less, and it contains more than 90% of toughened iron, Asada iron and At least one type of tempered Asada scattered iron; and the grain boundary solid solution ratio Z is 0.3 or more. The aforementioned grain boundary solid solution ratio Z is defined as Z = (mass% of Nb and Mo in grain boundaries. ) / (Mass% of one or two of Nb and Mo when dissolved). The hot-embossed molded body as claimed in claim 1, which has a plating layer.