TWI667351B - Hot stamping - Google Patents

Abstract

A hot stamping formed body having excellent impact absorption capability, characterized in that it has a predetermined composition; and the microstructure includes old Worthite iron having an average crystal grain size of 3 μm or less, and more than 90% by area ratio At least one of toughened iron, 麻田散铁, and tempered 麻田散铁, and the grain boundary solid solution ratio Z is above 0.3, and the grain boundary solid solution ratio Z is defined as Z = (Nb in the grain boundary and One or two mass% of Mo)/(% by mass of Nb and Mo or two kinds of masses at the time of dissolution).

Description

Hot stamping

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot stamping molded body which is used for a structural member or a reinforcing member of an automobile or structure requiring strength, and more particularly to a hot stamping molded body excellent in impact absorption capability.

Background Art In recent years, the weight of automobile bodies has been demanded from the viewpoint of environmental protection and resource conservation. Therefore, the application of high-strength steel sheets to components for automobiles has been accelerating. However, since the formability deteriorates as the strength of the steel sheet increases, the formability of the member having a complicated shape is a problem for the high-strength steel sheet.

In order to solve the above problems, the application of hot stamping for press forming after heating a steel sheet to a high temperature in the Worthfield Iron Zone is continuing. Since the hot stamping is subjected to the quenching treatment in the mold while being subjected to the press working, it has been attracting attention as a technique for achieving both the molding of the member for automobiles and the strength of the securing.

On the other hand, in a molded body obtained by molding a high-strength steel sheet by hot stamping, it is required to have an effect of absorbing an impact at the time of collision.

As a technique for satisfying this requirement, Patent Document 1 discloses annealing a steel sheet for hot stamping, and Mn or Cr is concentrated in a carbide to form a carbide which is hard to be dissolved, thereby utilizing it during hot stamping heating. These carbides inhibit the growth of the Worthite iron to make it fine-grained.

Patent Document 2 discloses a technique in which Worstian iron is finely granulated by heating at a heating rate of 90° C./s or less at the time of hot embossing heating.

Patent Document 3, Patent Document 4, and Patent Document 5 also disclose a technique for fine-graining Worthite iron to improve toughness.

CITATION LIST Patent Literature Patent Literature 1: International Patent Publication No. 2015/147216 Patent Document 2: Japanese Patent No. 5,369,714 Patent Document 3: Japanese Patent No. 5114691 Patent Document 4: Japanese Patent Laid-Open No. 2014-15638 Patent Document 5: Japanese Patent Laid-Open Publication No. 2002-309345

Disclosure of the Invention Problems to be Solved by the Invention However, it is difficult to obtain a finer-grained Worthite iron by the techniques disclosed in Patent Documents 1 to 5, and it is impossible to obtain a shock absorption capability which is conventionally known or more.

In view of the problems of the prior art, the present invention has an object of ensuring more excellent impact absorption capability in a hot-embossed molded article of a high-strength steel sheet, and an object of the invention is to provide a hot stamping molded article which can solve the problem.

Means for Solving the Problem The present inventors have intensively studied how to solve the above problems. As a result, it was found that the embrittlement strength of the grain boundary was increased by making the particle size of the old Worthite iron 3 μm or less and dissolving one or both of Nb and Mo in the old Worthfield iron grain boundary. It can obtain better impact absorption capacity than ever before.

The invention of the present application has been made based on the above findings and further studied, and the gist thereof is as follows.

(1) A hot stamping molded article comprising: C: 0.15% or more and less than 0.35%, Si: 0.005% or more and 0.25% or less, and Mn: 0.5% or more in terms of mass%; 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, and 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, and N: 0.010% The following, and the remaining part is Fe and unavoidable impurities; and the microstructure includes the old Worthite iron having an average crystal grain size of 3 μm or less, and more than 90% of the toughened iron and the granulated iron in the area ratio And at least one of the tempered granulated iron, and the grain boundary solid solution ratio Z is 0.3 or more, and the grain boundary solid solution ratio Z is defined as Z = (the mass of one or two of Nb and Mo in the grain boundary) %) / (% by mass of Nb and Mo or 2 kinds of mass at the time of dissolution).

(2) The hot stamping formed body according to (1) above, which has a plating layer.

Advantageous Effects of Invention According to the present invention, it is possible to provide a hot stamping molded body having high strength and superior impact absorption capability than conventional ones.

EMBODIMENT OF THE INVENTION The present invention is characterized in that the average grain size of the old Worthite iron is 3 μm or less, and one or two kinds of Nb and Mo are dissolved in the old Worthfield iron grain boundary to make the grain boundary brittle. The intensity of the increase. As a result of intensive research, the inventors have found that the above-described organization can be obtained by the following method.

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

If the amount of molten steel casting 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 be segregated in the old Worthfield iron grain boundary. As a result, although the old Worthfield iron grain boundary has been finely granulated, the embrittlement strength of the grain boundary is lowered, and the impact absorption ability cannot be sufficiently obtained. This is because the affinity between Mn and P is high, and segregation of Mn exerts a function as a trap position of P, and elimination of segregation causes P to diffuse in the old Worthfield iron grain boundary. In the present invention, this problem is solved by controlling the rolling conditions of the second stage.

In the second stage, by controlling the rolling reduction rate, the temperature, the cooling condition after rolling, and the coiling temperature, the Mn is concentrated in the carbide to make the easily formed fine carbides. In turn, a high-density difference row is introduced into the steel. In the present invention, the old Worthfield iron particles are refined by both the finely dispersed carbide and the high-density difference row, which are the inverted state positions of the Worthite iron. In order to effectively function as a position of the inverter state, the carbide should be easily dissolved. For this reason, it is important that an element such as Mn or Cr which inhibits dissolution of the carbide is not concentrated in the carbide.

Further, precipitation of Mo and Nb is suppressed, and Nb and Mo are dissolved in the old Worthfield iron grain boundary, whereby Nb and Mo are used to occupy the segregation position of P, thereby eliminating the segregation of P to the old Worthite iron. Thereby, not only Mo or Nb can be used to increase the grain boundary strength, but also the grain boundary embrittlement strength can be suppressed from being lowered.

In the third stage, by controlling the rate of temperature rise during hot stamping, both the readily soluble fine carbides and the high-density difference rows become the nucleation sites of the old Worthite iron. Thereby, the average crystal grain size of the old Worthite iron in the hot stamping molded body can be controlled to 3 μm or less.

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

The impact absorption capacity can be evaluated by the brittle surface rate of the sand impact test. The difference in the brittle surface rate is due to the difference in grain boundary strength. The grain boundary strength is based on the microstructure or type of the formed body (Mita loose iron, tempered granulated loose iron, lower toughened iron, etc.), the average crystal grain size of the old Worthite iron, grain boundaries such as Nb or Mo. Determined by the concentration of solid solution elements.

Although the grain boundary strength can be increased by increasing the amount of solid solution at the grain boundary of Nb or Mo, Nb and Mo combine with C in steel at a temperature of 500 ° C or higher to easily form carbides, so continuous control must be continued. Casting, hot rolling, and manufacturing steps up to hot stamping to suppress precipitation of such elements. That is, in order to increase the grain boundary solid solution amount of Nb or Mo, it is necessary to satisfy the conditions described later in all the stages from the first stage to the third stage.

Hereinafter, the hot stamping molded body of the present invention and a method for producing the same will be described in detail.

First, the reason for limiting the chemical composition of the hot stamping molded body of the present invention will be described. Hereinafter, % related to the component composition means mass%.

"C: 0.15% or more and less than 0.35%" C is an important element for obtaining a tensile strength of 1500 MPa or more. If it is less than 0.15%, the granulated iron is soft, and it is difficult to ensure the tensile strength of 1500 MPa or more, so the C system is set to 0.15% or more. And it should be 0.20% or more. On the other hand, it is set to be less than 0.35% in view of the required balance of impact absorption capability and strength. And preferably less than 0.34%.

"Si: 0.005% or more and 0.25% or less" The Si system is an element that can improve the deformation ability and contribute to the improvement of the impact absorption ability. If it is less than 0.005%, the deformation ability is poor and the impact absorption ability is deteriorated, so 0.005% or more is added. And it is preferably 0.01% or more. On the other hand, when it is more than 0.25%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the average crystal grain size of the old Worthite iron cannot be controlled to 3 μm, so the upper limit is imposed. Set to 0.25%. And it should be 0.22% or less.

"Mn: 0.5% or more and 3.0% or less" Mn is an element which contributes to strength improvement by solid solution strengthening. If it is less than 0.5%, the solid solution strengthening ability is poor, and the granulated iron is softened, and it is difficult to ensure the tensile strength of 1500 MPa or more, so 0.5% or more is added. And it should be 0.7% or more. On the other hand, when the addition is more than 3.0%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the average crystal grain size of the old Worthite iron cannot be controlled to 3 μm or less. The upper limit is 3.0%. And it should be 2.5% or less.

"sol. Al: 0.0002% or more and 3.0% or less" The Al system is an element which functions to deoxidize the molten steel and to refine the steel. If it is less than 0.0002%, it will sufficiently deoxidize and form a coarse oxide to cause early fracture, so the sol. Al system is set to 0.0002% or more. And it should be 0.0010% or more. On the other hand, when the addition is more than 3.0%, coarse oxides are formed to impair the toughness, and therefore it is set to 3.0% or less. It is preferably 2.5% or less, more preferably 0.5% or less.

"Cr: 0.05% or more and 1.00% or less" Cr is an element which contributes to strength improvement by solid solution strengthening. If it is less than 0.05%, the solid solution strengthening ability is poor, and the granulated iron is softened, and it is difficult to ensure the tensile strength of 1500 MPa or more, so 0.05% or more is added. And it should be 0.1% or more. On the other hand, when the addition is more than 1.00%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the old Worthite iron particle size cannot be controlled to be 3 μm or less, so 1.00% is obtained. The upper limit. And it should be 0.8% or less.

"B: 0.0005% or more and 0.010% or less" B is an element which contributes to strength improvement by solid solution strengthening. If it is less than 0.0005%, the solid solution strengthening ability is poor, and the granulated iron is softened, and it is difficult to ensure the tensile strength of 1500 MPa or more, so it is necessary to add 0.0005% or more. And it should be 0.0008% or more. On the other hand, when the addition is more than 0.010%, the amount of solid solution in the carbide increases, the carbide becomes difficult to dissolve, and the average crystal grain size of the old Worthite iron cannot be controlled to 3 μm or less. The upper limit is 0.010%. And it should be 0.007% or less.

"Nb: 0.01% or more and 0.15% or less" Nb is an element which is dissolved in the old Worthfield iron grain boundary and increases the grain boundary strength. Further, since Nb is dissolved in the grain boundary, the grain boundary segregation of P can be inhibited, so that the embrittlement strength of the grain boundary can be enhanced. Therefore, it is necessary to add 0.01% or more. And it should be 0.030% or more. On the other hand, when it is added more than 0.15%, it is easily precipitated as a carbide, and the amount of solid solution at the grain boundary is lowered, so that it is 0.15% or less. And preferably below 0.12%.

"Mo: 0.005% or more and 1.00% or less" Mo is an element which is solid-solubilized in the old Worthfield iron grain boundary and increases the grain boundary strength. Further, since Mo is dissolved in the grain boundary to inhibit the segregation of the grain boundary of P, the embrittlement strength of the grain boundary can be enhanced. Therefore, it is necessary to add 0.005% or more. And it should be 0.030% or more. On the other hand, when it is more than 1.00%, it is easy to precipitate as a carbide, and the solid solution amount at the grain boundary is lowered, so it is 1.00% or less. And it should be below 0.80%.

"Ti: 0% or more and 0.15% or less" Although Ti is not an essential element, it is an element which can enhance the strength by solid solution strengthening, and therefore may be added as needed. When Ti is added, it is preferably set to 0.01% or more in order to obtain an additive effect. And preferably 0.02%. On the other hand, when the addition is more than 0.15%, coarse carbides or nitrides are formed to cause early fracture, so that it is made 0.15% or less. And it should be 0.12% or less.

"Ni: 0% or more and 3.00% or less" Ni is an element which is not essential, but is an element which can enhance strength by solid solution strengthening, and therefore may be added as needed. When Ni is added, it is preferably set to 0.01% or more in order to obtain an additive effect. And preferably 0.02%. On the other hand, if the addition is more than 3.00%, the steel becomes brittle and causes early fracture, so it is set to 3.00% or less. And it should be 2.00% or less.

"P: 0.10% or less" P is an element which is an impurity element and is likely to be segregated in the grain boundary to lower the embrittlement strength of the grain boundary. If it is more than 0.10%, the embrittlement strength of the grain boundary is remarkably lowered to cause early fracture, so the P system is set to be 0.10% or less. And preferably less than 0.050%. Although the lower limit is not particularly limited, if it is reduced to less than 0.0001%, the cost of P removal is greatly increased and it is disadvantageous to the economical surface. Therefore, 0.0001% is a practical lower limit on a practical steel plate.

"S: 0.10% or less" S is an element which is an impurity element and forms an inclusion. If it is more than 0.10%, inclusions are formed and early fracture occurs, so the S system is set to 0.10% or less. And 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 the S is greatly increased and it is disadvantageous to the economical surface. Therefore, on a practical steel sheet, 0.0015% is a substantial lower limit.

"N: 0.010% or less" N is an impurity element, and nitride is formed to cause early fracture. Therefore, 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 N removal is greatly increased and it is disadvantageous to the economical surface. Therefore, 0.0001% is a practical lower limit on a practical steel plate.

The remainder of the composition of the composition is Fe and impurities. As the impurities, elements which are tolerated from the steel raw materials or scraps and/or inadmissiblely in the steel making process and which do not hinder the characteristics of the hot stamping molded body of the present invention can be exemplified.

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

"The average crystal grain size of the old Worthite iron is 3.0 μm or less."

The average crystalline particle size of the old Vossian iron is an important tissue factor to ensure excellent strength and to ensure the effect of inhibiting early fracture. According to the study by the inventors of the present invention, in order to obtain the impact absorption capability required for the hot stamping molded body, the particle size of the old Worthite iron is preferably as small as possible, and the average crystal grain 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 practical operation, it is difficult to make it less than 0.5 μm, so 0.5 μm is the substantial lower limit.

The average crystal grain size of the old Worthite iron was measured as follows.

First, the hot stamping formed body was heat-treated at 540 ° C for 24 hr. In order to promote the corrosion of the old Worthfield iron grain boundary. The heat treatment may be carried out by furnace heating or electric heating, and 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 is cut out from the center portion of the hot stamping molded body after the heat treatment, and the measurement surface is ground using a carbonized paper of #600 to #1500, and then a diamond powder having a particle size of 1 to 6 μm is used in alcohol or the like. The liquid obtained by dispersing the diluted solution or pure water is 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 Worthfield iron grain boundary. At this time, the etching operation is performed in the exhaust gas treatment device, and the temperature of the working gas atmosphere is set to normal temperature.

After the corroded sample was washed with acetone or ethanol, it was dried and observed by a scanning electron microscope. The scanning electron microscope used was set to be equipped with two electron detectors.

In a vacuum of 9.6 × 10 ^-5 or less, the sample was irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13, and a position of 1/8 to 3/8 was taken centering on the position of the 1/4 plate thickness of the sample. Secondary electronic image of the range. The shooting magnification is set to 4000 times based on the screen of 386 mm horizontally and 290 mm vertical, and the number of shooting visual fields is set to 10 or more.

In the secondary electronic image taken, the old Worthfield iron grain boundary was photographed as a bright contrast. The average value of the shortest diameter and the longest diameter was calculated from the old Worthfield iron particles contained in the observation field of view and set as the average crystal grain size. Except for the old Worthite iron particles, which are not included in the field of view, and the entire Worstian iron particles in the field of view, the above-mentioned operation is performed for all the old crystal grains in the photographing field. path. The average crystal grain size is a value obtained by dividing the sum of the calculated particle diameters by the total number of old Worthfield iron particles having the measured particle diameter. This operation was performed for all fields of view taken for each shot to calculate the average crystal grain size of the old Worthite iron.

"The solid solution ratio Z of the grain boundary defined by the formula (1) is 0.3 or more"

Z = 1% or 2% of Nb and Mo in the grain boundary, and 1% or 2% of Mo, or % by mass in dissolution... (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 ability, and is an index used by the inventors to evaluate the impact absorption ability. When Nb and/or Mo is dissolved in the grain boundary, P becomes difficult to segregate in the grain boundary and the bonding force of the grain boundary can be increased, so that the embrittlement strength of the grain boundary increases, and the impact absorption ability is improved. When 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 desired impact absorption ability cannot be obtained. Therefore, the grain boundary solid solution ratio Z system is set to 0.3 or more. And it should 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 system was measured as follows.

A test piece of the size shown in Fig. 1 was produced from the central portion of the hot stamping molded body. At this time, the front and back surfaces of the test piece were removed by mechanical grinding to the same amount each time, so that the plate thickness became 1.2 mm. The slit at the center of the test piece was inserted with a 1 mm thick wire cutter, and the joint at the bottom of the slit was controlled to be 100 μ to 200 μm.

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

The front and back surfaces of the test piece were subjected to galvanization within 0.5 hr after the completion of the impregnation.

After plating, it was supplied to Auger's electronic luminescence spectrometry within 1.5 hr. There is no particular limitation on the types of devices used to implement Auger's electronic luminescence spectroscopic analysis. The test piece was placed in an analysis apparatus, and was broken from the slit portion of the test piece in a vacuum of 9.6 × 10 ^-5 or less to expose the old Worthite iron grain boundary. An electron beam is irradiated to the exposed old Worthfield iron grain boundary at an acceleration voltage of 1 to 30 kV, and the mass % (concentration) of Nb and/or Mo in the grain boundary is measured. The measurement system was carried out in the old Worthfield iron grain boundary of 10 or more. In order to prevent contamination of the grain boundaries, the measurement is 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 mass % of Nb and/or Mo added is defined as the grain boundary solid solution ratio Z. .

"In terms of micro-area area ratio, more than 90% are one or more of the lower toughening iron, the granulated iron and the tempered granulated iron."

In order to obtain a tensile strength of 1500 MPa or more in the hot stamping formed body, the microstructure must contain more than 90% of the granulated iron or the tempered granulated iron in the area ratio. And it should be 94% or more. In terms of ensuring tensile strength, the microstructure may also be a lower toughened iron. The tissue having an area ratio of 90% or more may be one of the lower toughened iron, the granulated iron and the tempered granulated iron, and may be a mixed structure of the above.

The remainder of the microstructure is not specifically defined, and the above-mentioned toughened iron, residual Worthite iron, and Bora iron can be cited.

The area ratio of the lower toughened iron, the granulated iron, and the tempered granulated iron is determined as follows.

A cross section perpendicular to the plate surface is cut out from the center of the hot stamping formed body, and the measurement surface is ground using a carbonized crepe paper of #600 to #1500, and a diamond powder having a particle size of 1 to 6 μm is used in a diluent such as alcohol or pure. The liquid obtained by dispersing 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 boundary. At this time, the etching operation is performed in the exhaust gas treatment device, and the temperature of the working gas atmosphere is set to normal temperature.

After the corroded sample was washed with acetone or ethanol, it was dried and observed by a scanning electron microscope. The scanning electron microscope used was set to be equipped with two electron detectors. In a vacuum of 9.6 × 10 ^-5 or less, the sample was irradiated with an electron beam at an acceleration voltage of 10 kV and an irradiation current level of 8, and a position of 1/8 to 3/8 was taken centering on the position of the 1/4 plate thickness of the sample. Secondary electronic image of the range. The imaging magnification is set to 10000 times based on the screen of 386 mm in width × 290 mm in length, and the number of imaging fields is set to 10 or more.

In the secondary electron image obtained by photographing, since the crystal grain boundary and the carbide system are photographed as bright contrast, the structure can be easily determined by the position of the crystal grain boundary and the carbide. When carbides are formed inside the grains, the granulated loose iron or the lower toughened iron is tempered, and the structure in which no carbides are observed inside the grains is the granulated iron.

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

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

The photographing magnification was taken at 10,000 times on the basis of a screen of 386 mm in length and 290 mm in length, and a grid of 2 μm intervals was formed on the photographs taken, and the microstructure at the intersection of the grids was selected. The value obtained by dividing the number of intersections of each organization by all the intersection points is taken as the area ratio of the microstructure. This operation was performed in 10 fields of view, and the average value was calculated and used as the area ratio of the microstructure.

Next, the form of the hot stamping molded body of the present invention and the method for producing the hot stamping steel sheet used for the production of the hot stamping molded body will be described.

<Method for Producing Steel Sheet for Hot Stamping>

(1) Continuous casting step A molten steel having the above chemical composition is formed into a steel sheet (steel blank) by a continuous casting method. In this continuous casting step, it is preferred to set the amount of molten steel casting per unit time to 6 ton/min or less. In the continuous casting, if the casting amount per unit time (casting speed) of the molten steel is more than 6 ton/min, the nucleation amount of the precipitate mainly composed of Mo or Nb increases while the microsegregation of Mn increases. The casting amount is preferably set to 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.

(2) Hot rolling step The steel sheet was hot rolled to form a steel sheet. At this time, the hot rolling is terminated in a temperature zone of the A3 metamorphic temperature +10 ° C or more and the A3 metamorphic temperature +200 ° C or less defined by the formula (2), and the final section at this time is set to have a rolling reduction ratio of 12% or more. Cooling starts within 1 second from the end of the completion rolling, and is cooled at a cooling rate of 100 ° C/s or more in a temperature range from the completion rolling temperature to 550 ° C, and at a temperature of less than 500 ° C. Take a roll.

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

Recrystallization of the Worthite iron is promoted by setting the finishing rolling temperature to A3 metamorphic temperature + 10 °C or higher. Thereby, the formation of a low-angle grain boundary in the crystal grain is suppressed, and the precipitation position of Nb and Mo can be reduced. Further, since the consumption of C can be suppressed by reducing the precipitation positions of Nb and Mo, the number density of carbides can be increased in the subsequent steps. Preferably, the A3 metamorphic temperature is +30 ° C or higher.

The excessive grain growth of the Worthite iron is suppressed by setting the finishing rolling temperature to A3 metamorphic temperature + 200 ° C or lower. By performing the rolling in the temperature zone of A3 metamorphic temperature +200 ° C or lower, the recrystallization of the Worthite iron can be promoted, and excessive grain growth does not occur, so that fineness can be obtained in the coiling step. carbide. Preferably, the A3 metamorphic temperature is +150 ° C or less.

The recrystallization of the Worthite iron is promoted by setting the rolling reduction ratio of the finished rolling to 12% or more. Thereby, the formation of a low-angle grain boundary in the crystal grain is suppressed, and the precipitation position of Nb and Mo can be reduced. It is preferably at least 15%.

The cooling is started within 1 second from the end of the completion of the rolling and is preferably within 0.8 seconds, and is cooled at a cooling rate of 100 ° C/s or more in a temperature range from the completion rolling temperature to the temperature of 550 ° C. This can reduce the residence time in the temperature zone which promotes the precipitation of Nb and Mn. As a result, precipitation of Nb and Mo in the Worthite iron can be suppressed, and the amount of Nb and Mo solid solution in the Worthfield iron grain boundary increases.

When the coiling temperature is set to less than 500 ° C, the above effect is enhanced, and Mn is inhibited from being concentrated in the carbide, and fine carbides which are easily dissolved are formed, and a high-density difference row is introduced into the steel. It is preferably less than 480 °C. Although the lower limit is not particularly specified, it is difficult to perform the coiling at room temperature or lower in actual operation, so the room temperature is the lower limit.

(3) Formation of plating layer A plating layer is formed on the surface of the steel sheet for the purpose of improving corrosion resistance and the like. The plating layer may be either a plating layer or a molten plating layer. As the plating layer, an electrogalvanized layer, a plated Zn-Ni alloy layer, or the like can be exemplified. Examples of the molten plating layer include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminized layer, a molten Zn-Al alloy plating layer, a molten Zn-Al-Mg alloy plating layer, and a molten Zn-Al-Mg-Si alloy. Plating, etc. The amount of the plating layer to be attached 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 known methods such as pickling, cold rolling, and temper rolling are also included.

<Manufacturing procedure of hot stamping molded body>

The hot stamping formed body of the present invention is heated and maintained in a temperature range of 500 ° C or more and A3 or less in an average heating rate of 100 ° C / s or more and less than 200 ° C / s. Thereafter, hot stamping is performed, and after the molding, the molded body is cooled to room temperature to be produced.

Further, in order to adjust the strength, a part or all of the hot stamping molded body may be tempered at a temperature of 200 ° C or more and 500 ° C or less.

Heating, maintaining, and performing hot stamping 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 or less, thereby making it easy to dissolve fine carbides Both of the high-density difference rows are the nucleation sites of the old Worthite iron, and the average crystal grain size of the old Worthite iron can be controlled to be 3 μm or less. Further, it contributes to suppressing the precipitation of NbC and MoC during heating, and increases the solid solution ratio of one or two kinds of Nb and Mo in the old Worthite iron grain boundary.

The average heating rate is preferably 120 ° C / s or more. When the average heating rate is more than 200 ° C / s, the metamorphosis to the Vostian iron is promoted in the state where the dissolution of the carbide is not completed, and the toughness is deteriorated, so the upper limit is 200 ° C / s. And preferably less than 180 ° C / s.

The holding temperature at the time of hot stamping should be set to A3 point + 10 ° C or more and A3 point + 150 ° C or less. Further, the cooling rate after hot stamping is preferably set to 10 ° C / s or more.

EXAMPLES Next, the examples of the present invention are described, but the conditions in the examples are merely a conditional example used to confirm the practicability and effects of the present invention, and the present invention is not limited by the conditions. The present invention can adopt various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention.

The steel sheets obtained by casting the molten steel of the composition shown in Tables 1-1 to 1-3 are subjected to hot rolling and cold rolling to the conditions shown in Tables 2-1 to 2-3 for hot stamping. After the steel sheet, the heat-sensitive embossing steel sheets obtained were subjected to heat treatment shown in Tables 2-1 to 2-3 and hot-embossed to produce a molded body.

The microstructure and mechanical properties of the hot stamping formed body are shown in Tables 3-1 to 3-3.

[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]

Further, the tensile strength of the hot stamping molded article was measured by No. 5 test piece described in JIS Z 2201, and measured in accordance with the test method described in JIS Z 2241. As an indicator of impact absorption capacity, toughness was evaluated by a sand impact test. A sub-size sand impact test was carried out at -100 ° C, and the case where the brittle surface rate was less than 30% was acceptable.

It was confirmed that the hot-embossed molded article of the present invention has a tensile strength of 1,500 MPa or more and an excellent characteristic that the embrittlement index is less than 30%. On the other hand, in the case where the chemical composition and the manufacturing method are not appropriate, 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 stamping molded article comprising: C: 0.15% or more and less than 0.35%, Si: 0.005% or more and 0.25% or less, Mn: 0.5% or more, and 3.0% by mass%; Hereinafter, 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, and N: 0.010% or less. And the remaining part is Fe and the unavoidable impurities; and the microstructure contains the old Worthite iron with an average crystal grain size of 3 μm or less, and more than 90% of the toughened iron, the granulated iron and the back are included in the area ratio. At least one type of loose iron in the fire field, and the solid solution ratio Z of the grain boundary is 0.3 or more, and the solid solution ratio Z of the grain boundary is defined as Z = (% by mass of Nb and Mo in the grain boundary or mass% of two) / (1% or 2% of Nb and Mo in the case of dissolution). A hot stamping formed body according to claim 1, which has a plating layer.