HOT-STAMPED PRODUCT AND METHOD OF PRODUCING THE SAME

- NIPPON STEEL CORPORATION

A hot-stamped product 10 includes a first steel material 12 that includes a first plate-shaped portion 24 and a burring portion 26 rising from the first plate-shaped portion 24, a second steel material 14 that includes a second plate-shaped portion 44 provided so as to be aligned with the first plate-shaped portion 24 in a direction perpendicular to a thickness direction of the first plate-shaped portion 24, and a welding seam 16 that joins an edge of the first plate-shaped portion 24 and an edge of the second plate-shaped portion 44.

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Description
TECHNICAL FIELD

The present invention relates to a hot-stamped product and a method of producing the same.

BACKGROUND ART

In order to simultaneously reduce weight and ensure collision safety of automobiles, there is a need to increase the strength of materials used for automobiles. Therefore, in recent years, hot-stamped steels up to 1.8 GPa class have been put into practical use for vehicle bodies. In addition, if chassis parts are also required to increase the strength as in vehicle bodies, it will be necessary to consider utilization of hot-stamped steels in the future.

In the automobile parts as described above, burring processing is sometimes performed to assemble the parts, and the like. For example, Patent Document 1 discloses a method of producing a hot-pressed product that includes a burring portion.

The method of producing disclosed in Patent Document 1 includes a heating process of heating a plate material, and a forming step of forming the heated plate material. In the forming step, quenching and burring processing are performed in addition to the forming of the plate material.

LIST OF PRIOR ART DOCUMENTS Patent Document

    • Patent Document 1: JP2019-58916A

SUMMARY OF INVENTION Technical Problem

The present inventors conducted a detailed investigation of a burring portion formed by hot stamping (hot press forming), and found that the mechanical properties of the burring portion may differ from those of the other portions. On the other hand, the mechanical properties required for automobile parts generally are different depending on the applications of the automobile parts. Therefore, when using a hot stamped product as an automobile part, it is necessary to select a starting material so that both the burring portion and the portions other than the burring portion satisfy the required mechanical properties. In this case, material costs will increase.

In addition, depending on automobile parts, the mechanical properties required for a burring portion may be different from the mechanical properties required for the portions other than the burring portion. However, in conventional hot stamping as described above, it is difficult to separately change the mechanical properties of a burring portion and the mechanical properties of the portions other than the burring portion according to needs.

Therefore, an objective of the present invention is to provide a hot stamped product with high design freedom, and a method of producing the same.

Solution to Problem

The gist of the present invention is the following hot stamped product and a method of producing the same.

(1) A hot-stamped product including

    • a first steel material that includes a first plate-shaped portion and a burring portion rising from the first plate-shaped portion,
    • a second steel material that includes a second plate-shaped portion provided so as to be aligned with the first plate-shaped portion in a direction perpendicular to a thickness direction of the first plate-shaped portion, and
    • a welding seam that joins an edge of the first plate-shaped portion and an edge of the second plate-shaped portion.

(2) The hot-stamped product according to (1) described above, in which a Vickers hardness of a leading end portion of the burring portion and a Vickers hardness of the first plate-shaped portion are equal to or more than a Vickers hardness of the second plate-shaped portion.

(3) The hot-stamped product according to (1) or (2) described above, in which the Vickers hardness of the leading end portion of the burring portion is equal to or more than the Vickers hardness of the first plate-shaped portion.

(4) The hot-stamped product according to (2) or (3) described above, in which a first test specimen and a second test specimen are cut out from the second plate-shaped portion, and

    • a Vickers hardness HV1 of the first test specimen in a case where quenching is performed after applying strain to the first test specimen, and a Vickers hardness HV2 of the second test specimen in a case where quenching is performed without applying strain to the second test specimen satisfy the following formula (i):


HV1<HV2  (i).

(5) The hot-stamped product according to (2) or (3) described above, in which a third test specimen and a fourth test specimen are cut out from the first plate-shaped portion, and

    • a Vickers hardness HV3 of the third test specimen in a case where quenching is performed after applying strain to the third test specimen, and a Vickers hardness HV4 of the fourth test specimen in a case where quenching is performed without applying strain to the fourth test specimen satisfy the following formula (ii):


HV3≥HV4  (ii).

(6) The hot-stamped product according to (2) or (3) described above, in which a thickness of the leading end portion of the burring portion is smaller than a thickness of the first plate-shaped portion, and

    • a difference D1 between the thickness of the leading end portion of the burring portion and the thickness of the first plate-shaped portion, and a difference D2 between the thickness of the first plate-shaped portion and a thickness of the second plate-shaped portion satisfy the following formula (iii):


D1>D2  (iii).

(7) The hot-stamped product according to (1) or (2) described above, in which a Vickers hardness of a leading end portion of the burring portion is smaller than a Vickers hardness of the first plate-shaped portion.

(8) A method of producing a hot-stamped product according to any of (1) to (7) described above, the method including

    • a step of obtaining a tailored blank by welding a first steel sheet and a second steel sheet,
    • a step of heating the tailored blank,
    • a step of performing press forming on the tailored blank,
    • a step of performing burring processing on the first steel sheet of the heated tailored blank, and
    • a step of performing quenching on the heated tailored blank.

(9) The method of producing the hot-stamped product according to (8) described above, in which, in the step of obtaining the tailored blank, a metal coated steel sheet is used for at least one of the first steel sheet and the second steel sheet.

Advantageous Effects of Invention

According to the present invention, a hot stamped product with high design freedom can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows schematic diagrams illustrating hot stamped product according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a test piece.

 DESCRIPTION OF EMBODIMENTS

Hereinafter, a hot-stamped product and a method of producing for the same according to embodiments of the present invention will be described by using the drawings.

(Basic Configuration of Hot-stamped Product)

FIG. 1 shows schematic diagrams illustrating a hot-stamped product according to one embodiment of the present invention, in which the view (a) is a plan view, the view (b) is a sectional view of a b-b portion in the view (a), and the view (c) is a sectional view of a c-c portion in the view (a). As illustrated in FIG. 1, a hot-stamped product 10 (hereinafter abbreviated as the formed product 10) includes a first steel material 12, a second steel material 14, and a welding seam 16 that joins an edge of the first steel material 12 and an edge of the second steel material 14. The thickness of the first steel material 12 and the second steel material 14 is, for example, 1.2 to 4.0 mm.

As a starting material of the first steel material 12 and the second steel material 14, it is possible to use a steel consisting of, for example, by mass %, C: 0.10 to 0.50%, Si: 0.01 to 1.30%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.020% or less, N: 0.020% or less, Al: 0.10% or less, Nb: 0 to 0.20%, Ti: 0 to 0.20%, Cr: 0.010 to 1.00%, Mo: 0 to 1.00%, B: 0 to 0.010%, Co: 0 to 4.0%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, V: 0 to 1.0%, W: 0 to 1.0%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.010%, Sb: 0 to 0.10%, and Zr: 0 to 0.10%, with the balance consisting of Fe and impurities.

Note that impurities means components that are mixed in due to raw materials such as ores and scraps or other factors when industrially producing steel, and that are allowed within the range that does not have an adverse effect on the steel according to the present invention.

The first steel material 12 includes a pair of first vertical wall portions 20 and 22, a first plate-shaped portion 24 that connects the first vertical wall portion 20 and the first vertical wall portion 22, and a burring portion 26 rising from the first plate-shaped portion 24. A height L (the length in the thickness direction of the first plate-shaped portion 24) of the burring portion 26 is, for example, 3 to 30 mm, and the diameter of a circle inscribed in the leading end of the burring portion 26 when viewed from the thickness direction of the first plate-shaped portion 24 is, for example, 15 to 100 mm.

The second steel material 14 includes a pair of second vertical wall portions 40 and 42, and a second plate-shaped portion 44 that connects the second vertical wall portion 40 and the second vertical wall portion 42. In the present embodiment, the first vertical wall portion 20 and the second vertical wall portion 40 are joined, the first vertical wall portion 22 and the second vertical wall portion 42 are joined, and the first plate-shaped portion 24 and the second plate-shaped portion 44 are joined by the welding seam 16. The first plate-shaped portion 24 and the second plate-shaped portion 44 are arranged so as to be aligned in a direction perpendicular to the thickness direction of the first plate-shaped portion 24.

Note that, in FIG. 1, although the burring portion 26 is formed so as to rise perpendicularly to the first plate-shaped portion 24, the burring portion 26 may rise so as to be inclined with respect to the first plate-shaped portion 24. For example, the burring portion 26 may be formed such that the diameter becomes smaller toward the leading end portion side (as the distance from the first plate-shaped portion 24 in the thickness direction of the first plate-shaped portion 24 is increased).

In addition, in FIG. 1, the burring portion 26, the first vertical wall portion 20, the first vertical wall portion 22, the second vertical wall portion 40, and the second vertical wall portion 42 are formed so as to rise from the first plate-shaped portion 24 to one side in the thickness direction of the first plate-shaped portion 24. However, the rising direction of the burring portion and the vertical wall portions is not limited to the example described above. For example, in the thickness direction of the first plate-shaped portion 24, the burring portion 26 may be formed so as to rise in the opposite direction to the first vertical wall portions 20 and 22 and the second vertical wall portions 40 and 42. In addition, in the thickness direction of the first plate-shaped portion 24, the first vertical wall portion 20 and the second vertical wall portion 40 may be formed so as to rise in the opposite direction to the first vertical wall portion 22 and the second vertical wall portion 42.

In addition, in FIG. 1, when viewed from the thickness direction of the first plate-shaped portion 24, although the first vertical wall portions 20 and 22, the first plate-shaped portion 24, the second vertical wall portions 40 and 42, and the second plate-shaped portion 44 are formed so as to extend linearly, when viewed from the thickness direction of the first plate-shaped portion 24, the first vertical wall portions 20 and 22, the first plate-shaped portion 24, the second vertical wall portions 40 and 42, and the second plate-shaped portion 44 may be curved.

In addition, in FIG. 1, although the first plate-shaped portion 24 and the second plate-shaped portion 44 have flat plate shapes, the shapes of the first plate-shaped portion 24 and the second plate-shaped portion 44 are not limited to the flat plate shapes. For example, when viewed from a direction perpendicular to the thickness direction of the first plate-shaped portion 24, the first plate-shaped portion 24 and the second plate-shaped portion 44 may be curved, and unevenness may be formed in the first plate-shaped portion 24 and the second plate-shaped portion 44.

In addition, in FIG. 1, although the burring portion 26 has a cylindrical shape, the burring portion 26 may be formed in a tube shape, and may have, for example, a square tube shape. In addition, a notch may be formed in a part of the burring portion 26. For example, the burring portion 26 may not have a complete tube shape, and may have a C-shape in a cross section perpendicular to a tube axis direction. In this description, a burring portion refers to a portion that has a tube shape or a substantially tube shape, and to which a forming strain is given to particularly a leading end portion side by forming the leading end portion such that the leading end portion is pushed outward. However, the present invention can be preferably utilized in a formed product that has a burring portion at least partially having a circular cross section (the cross section perpendicular to a tube axis direction).

Note that the formed product 10 illustrated in FIG. 1 is merely an example, and the present invention can be applied to various shapes of hot-stamped products that include a first steel material that includes a burring portion rising from a first plate-shaped portion, a second steel material that includes a second plate-shaped portion provided so as to be aligned with the first plate-shaped portion, and a welding seam that joins the first plate-shaped portion and the second plate-shaped portion. Accordingly, for example, the formed product 10 may further include a steel material (a patch member welded to the first steel material or the second steel material, or the like) for reinforcement, in addition to the first steel material and the second steel material. Although a detailed description will be omitted, a hot-stamped product according to the present invention is used as, for example, a chassis part (a cross member, a lower arm, a torsion beam, or the like) of an automobile.

(Summary of Method of Producing Hot-Stamped Product)

A method of producing a formed product according to the present embodiment includes a first step of obtaining a tailored blank, a second step of heating the tailored blank, a third step of performing press forming on the tailored blank, a fourth step of performing burring processing on the first steel sheet, and a fifth step of quenching the heated tailored blank. Hereinafter, taking a case where the formed product 10 is produced as an example, a method of producing a formed product according to the present embodiment will be described.

In the first step, a first steel sheet (not illustrated) that serves as the first steel material 12 in the formed product 10, and a second steel sheet (not illustrated) that serves as the second steel material 14 in the formed product 10 are butt-welded together. Note that the first steel sheet and the second steel sheet are joined together by utilizing a known welding method, such as laser welding or plasma arc welding, as in a known tailor welded blank (TWB). In addition, although a detailed description will be omitted, a pilot hole for forming the burring portion 26 is formed in the first steel sheet.

The heating conditions in the second step are not particularly limited. In the present embodiment, for example, a tailored blank (the first steel sheet and the second steel sheet) is heated to a temperature of the Ac3 point or more. The upper limit of the heating temperature of the tailored blank is not particularly limited, and may be appropriately set according to a material used. Note that the Ac3 point is calculated by the following formula (a).

Ac 3 = 850 + 1 ⁢ 0 ⁢ ( C + N ) ⁢ Mn + 350 ⁢ Nb + 250 ⁢ Ti + 4 ⁢ 0 ⁢ B + 10 ⁢ Cr + 100 ⁢ Mo ( a )

    • where each element symbol in the formula represents the content (mass %) of each element included in the steel sheet, and is set to zero when the element is not contained.

In the third step, press forming is performed on the heated tailored blank, so as to form the first vertical wall portions 20 and 22, the first plate-shaped portion 24, the second vertical wall portions 40 and 42, and the second plate-shaped portion 44. In the fourth step, burring processing is performed on the first steel sheet of the heated tailored blank, so as to form the burring portion 26. Note that the press forming in the third step and the burring processing in the fourth step are preferably started at 600° C. or more. In the fifth step, quenching is performed by rapidly cooling the tailored blank (the first steel sheet and the second steel sheet) to a temperature of the Mf point or less. In the quenching in the fifth step, the tailored blank (the first steel sheet and the second steel sheet) is held in a die set, and is cooled to a temperature of the Mf point or less by transferring heat to the die set. The cooling rate in the fifth step is set to, for example, 20° C./s or more. Then, the tailored blank cooled to the temperature of the Mf point or less is allowed to cool to room temperature. Note that the Mf point is calculated by the following formula (b).

Mf = 410.5 - 407.3 C - 7.3 Si - 37.8 Mn - 20.5 Cu - 19.5 Ni - 19.8 Cr - 4.5 Mo ( b )

    • where each element symbol in the formula represents the content (mass %) of each element included in the steel sheet, and is set to zero when the element is not contained.

In the present embodiment, the third step, the fourth step, and the fifth step are performed as a series of steps. For example, after heating a tailored blank in the second step, press forming (the third step) and burring processing (the fourth step) may be performed at the same time, and then quenching (the fifth step) may be performed without releasing the tailored blank from the die set. In addition, for example, after heating the tailored blank in the second step, press forming (the third step) and burring processing (the fourth step) may be successively performed without releasing the tailored blank from the die set, and then quenching (the fifth step) may be performed without releasing the tailored blank from the die set.

Note that press forming (the third step) and burring processing (the fourth step) may be performed as separate steps. For example, after performing forming (the third step) and quenching (the fifth step) of the first vertical wall portions 20 and 22, the first plate-shaped portion 24, the second vertical wall portions 40 and 42, and the second plate-shaped portion 44 as a series of steps, burring processing (the fourth step) and quenching (the fifth step) may be performed as a series of steps. In this case, the second step (heating) is performed before performing each of the third step and the fourth step. That is, the second step and the fifth step are each performed twice. For example, after performing cold press forming (the third step), heating (the second step) of the tailored blank may be performed, and then burring processing (the fourth step) and quenching (the fifth step) may be performed as a series of steps. That is, after the first step is performed, the third step may be performed before performing the second step. Note that a known method can be utilized for press forming and burring processing.

Note that, in the present embodiment, hot stamping (hot press forming) is performed in the third step as described above. In order for quenching to be appropriately performed at this time, Ceq (carbon equivalent) of the first steel material 12 (the first steel sheet) and the second steel material 14 (the second steel sheet) defined by the following formula (c) is preferably 0.35 or more.

Ceq = C + Si / 2 ⁢ 4 + M ⁢ n / 6 + N ⁢ i / 4 ⁢ 0 + C ⁢ r / 5 + M ⁢ o / 4 + V / 14 ( c )

    • where each element symbol in the formula represents the content (mass %) of each element included in the steel material (steel sheet), and is set to zero when the element is not contained.

(Regarding Combination of First Steel Material and Second Steel Material)

The present inventors found out that a function that could not be achieved in conventional hot-stamped products could be given to a hot-stamped product by appropriately selecting a starting material of the first steel material (the first steel sheet) or the second steel material (the second steel sheet). Hereinafter, a combination of the first steel material and the second steel material and the effects thereof will be described. Note that, in the present embodiment, the Vickers hardness (HV1) of the leading end portion of the burring portion, the Vickers hardness (HV1) of the first plate-shaped portion, and the Vickers hardness (HV1) of the second plate-shaped portion are each preferably 280 or more, 360 or more, 450 or more, 540 or more, 580 or more, or 650 or more. In addition, the Vickers hardness (HV1) of the leading end portion of the burring portion, the Vickers hardness (HV1) of the first plate-shaped portion, and the Vickers hardness (HV1) of the second plate-shaped portion are each 760 or less, for example. Note that “HV1” means the “hardness symbol” in a case where a Vickers hardness test is performed with a test force of 9.8 N (1 kgf) (refer to JIS Z 2244-1:2020). In this description, when Vickers hardness is simply written, it means Vickers hardness (HV1).

(Example 1) The starting material of the first steel material and the second steel material is selected so that the Vickers hardness of the leading end portion of the burring portion and the Vickers hardness of the first plate-shaped portion become the Vickers hardness of the second plate-shaped portion or more.

When utilizing the formed product 10 as an automobile part, it is conceivable that another part is attached to the burring portion 26. In this case, the leading end portion of the burring portion 26 and the periphery of the burring portion 26 are required to have excellent strength. In addition, the leading end portion of the burring portion 26 and the periphery of the burring portion 26 may be required to have wear resistance. In such a case, the starting material of the first steel material 12 and the second steel material 14 are selected so that the Vickers hardness of the leading end portion of the burring portion 26 and the Vickers hardness of the first plate-shaped portion 24 become the Vickers hardness of the second plate-shaped portion 44 or more. That is, a steel whose hardness becomes the hardness of the second steel material 14 or more by quenching is used as the starting material of the first steel material 12. Accordingly, compared with a case where an entire formed product is constituted from a high-strength material, the strength required of a formed product can be ensured, while suppressing the production costs of the formed product. In addition, the wear resistance of the leading end portion of the burring portion 26 and the periphery of the burring portion 26 can also be ensured.

Note that the Vickers hardness of the leading end portion of the burring portion 26 is measured in a cut surface of the burring portion 26 (in the present embodiment, a cut surface obtained by cutting the burring portion 26 in half) that passes through the axial centerline of the burring portion 26 and is parallel to the axial centerline, along the height direction of the burring portion 26 with the measurement center being set at a position 5 mm from the leading end of the burring portion 26 toward the first plate-shaped portion 24. Specifically, measurement is performed at five locations at a T/4 (T is the thickness at the Vickers hardness measurement position of the leading end portion of the burring portion 26) depth position from an outer periphery surface 26a of the burring portion 26, with a test force (measurement load) of 9.8 N (1 kgf), a pitch of 0.5 mm, the center being set at a position 5 mm from the leading end of the burring portion 26 toward the first plate-shaped portion 24. The average value of the measured values is used as the Vickers hardness of the leading end portion of the burring portion 26. Note that the thickness of the burring portion 26 is measured as the distance between the outer periphery surface 26a and an inner periphery surface 26b measured in a direction perpendicular to the outer periphery surface 26a on the basis of the outer periphery surface 26a.

In addition, the Vickers hardness of the first plate-shaped portion 24 is measured at five locations in a cross section parallel to the thickness direction of the first plate-shaped portion 24 at a position that is sufficiently distant from the welding seam 16 and the burring portion 26, at a t1/4 (t1 is the thickness at the Vickers hardness measurement position of the first plate-shaped portion 24) depth position from a surface (a surface on which the burring portion 26 is formed) of the first plate-shaped portion 24, with the test force described above and a pitch of 0.5 mm. The average value of the measured values is used as the Vickers hardness of the first plate-shaped portion 24. Note that the position that is sufficiently distant from the welding seam 16 is a position 3 mm or more distant from the boundary between the welding seam 16 and the first steel material 12 in a case where the first steel material 12 and the second steel material 14 are welded together by laser welding, and is a position 10 mm or more distant from the boundary in a case where the first steel material 12 and the second steel material 14 are welded together by arc welding. In addition, the position that is sufficiently distant from the burring portion 26 is a position that is 10 mm or more distant from the rising position of the burring portion 26.

Similarly, the Vickers hardness of the second plate-shaped portion 44 is measured at five locations in a cross section parallel to the thickness direction of the second plate-shaped portion 44 at a position that is sufficiently distant from the welding seam 16, at a t2/4 (t2 is the thickness at the Vickers hardness measurement position of the second plate-shaped portion 44) depth position from a surface of the second plate-shaped portion 44 (a surface that is continuous with the surface on which the burring portion 26 of the first plate-shaped portion 24 is formed. A bottom surface of the second plate-shaped portion 44 in FIG. 1 (c)), with the test force described above and a pitch of 0.5 mm. The average value of the measured values is used as the Vickers hardness of the second plate-shaped portion 44. Note that the position that is sufficiently distant from the welding seam 16 is a position 3 mm or more distant from the boundary between the welding seam 16 and the second steel material 14 in a case where the first steel material 12 and the second steel material 14 are welded together by laser welding, and is a position 10 mm or more distant from the boundary in a case where the first steel material 12 and the second steel material 14 are welded together by arc welding.

Example 1a

In (Example 1), for example, the starting material of the first steel material 12 and the second steel material 14 may be selected so that the carbon content of the first steel material 12 becomes greater than the carbon content of the second steel material 14. Specifically, for example, the following steel A can be used as the starting material of the first steel material 12, and the following steel B can be used as the starting material of the second steel material 14.

(Steel A) A steel consisting of, by mass %, C: more than 0.35 to 0.50% or less, Si: 0.01 to 1.30%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.020% or less, N: 0.020% or less, Al: 0.10% or less, Nb: 0 to 0.20%, Ti: 0 to 0.20%, Cr: 0.010 to 1.00%, Mo: 0 to 1.00%, B: 0 to 0.010%, Co: 0 to 4.0%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, V: 0 to 1.0%, W: 0 to 1.0%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.010%, Sb: 0 to 0.10%, and Zr: 0 to 0.10%, with the balance consisting of Fe and impurities.

(Steel B) A steel consisting of, by mass %, C: 0.10 to 0.35%, Si: 0.01 to 1.30%, Mn: 0.10 to 3.00%, P: 0.050% or less, S: 0.020% or less, N: 0.020% or less, Al: 0.10% or less, Nb: 0 to 0.20%, Ti: 0 to 0.20%, Cr: 0.010 to 1.00%, Mo: 0 to 1.00%, B: 0 to 0.010%, Co: 0 to 4.0%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, V: 0 to 1.0%, W: 0 to 1.0%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, REM: 0 to 0.010%, Sb: 0 to 0.10%, and Zr: 0 to 0.10%, with the balance consisting of Fe and impurities.

Note that impurities means components that are mixed in due to raw materials such as ores and scraps or other factors when industrially producing steel, and that are allowed within the range that does not have an adverse effect on the steel according to the present invention.

Note that when the carbon content of the first steel material 12 is made greater than the carbon content of the second steel material 14 as described above, by increasing the proportion of the second steel material 14 in the formed product 10, the hydrogen embrittlement resistance of the formed product 10 can be improved, while ensuring the strength and the wear resistance that are required of the burring portion 26. In addition, the ductility and toughness can be ensured as the entire formed product 10 by increasing the proportion of the second steel material 14. Accordingly, the deformability of the formed product 10 can be improved. Note that the deformability of the formed product 10 means the ability (fracture resistance) to suppress fracture due to deformation. In addition, in this description, the content of each element of the first steel material 12 and the second steel material 14 means the content of each element of the steel materials from which a surface layer portion has been removed by polishing or the like, when a scale, a decarburization layer, a metal coating solid solution layer, and the like are formed in the surface layer portion of the steel materials.

Note that a steel with a higher carbon content and lower hardenability than the starting material of the second steel material 14 may be used as the starting material of the first steel material 12, and a steel with high hardenability may be used as the starting material of the second steel material 14. In this case, for example, the formed product 10 may be produced so that the Vickers hardness of the leading end portion of the burring portion 26 and the Vickers hardness of the first plate-shaped portion 24 have values equivalent to the Vickers hardness of the second plate-shaped portion 44. Accordingly, the formed product 10 with high homogeneity can be obtained. Note that, in this description, a steel with low hardenability means a steel having a Ceq (carbon equivalent) defined by the formula (c) of less than 0.65, and a steel with high hardenability means a steel having a Ceq of 0.65 or more. Accordingly, for example, a steel satisfying the conditions of the steel A described above and having a Ceq of less than 0.65 can be used as the starting material of the first steel material 12, and a steel satisfying the conditions of the steel B described above and having a Ceq of 0.65 or more can be used as the starting material of the second steel material 14.

Note that even when a steel with low hardenability is used, in hot stamping where a large strain is not applied, it is preferable that quenching is appropriately performed. That is, even when quenching is not appropriately performed in a portion subjected to processing such as burring processing in which a large strain is applied, it is preferable that quenching is appropriately performed in the other portions. Therefore, even for a steel with low hardenability, it is preferable that Ceq is 0.35 or more.

Example 1b

In addition, in (Example 1), both the first steel material 12 and the second steel material 14 may satisfy the conditions of the steel A described above, or both the first steel material 12 and the second steel material 14 may satisfy the conditions of the steel B described above. In these cases, for example, the carbon contents of the starting materials of the first steel material 12 and the second steel material 14 may be equivalent, and a steel with high hardenability may be used as the starting material of the first steel material 12, and a steel with low hardenability may be used as the starting material of the second steel material 14. Specifically, for example, a steel having a Ceq defined by the formula (a) of 0.65 or more may be used as the starting material of the first steel material 12, and a steel having a Ceq of less than 0.65 may be used as the starting material of the second steel material 14. In this case, for example, the Vickers hardness of the leading end portion of the burring portion 26 and the Vickers hardness of the first plate-shaped portion 24 can be set to values equivalent to the Vickers hardness of the second plate-shaped portion 44. Accordingly, the formed product 10 with high homogeneity can be obtained. In addition, by using a steel with high hardenability as the starting material of the first steel material 12, the burring portion 26 can be appropriately quenched without increasing the die-set cooling rate. Accordingly, since it is unnecessary to increase the cooling rate of the entire die set, or to partially increase the cooling rate of a portion of the die set that comes into contact with the burring portion 26, the production of the formed product 10 becomes easy. In addition, by using a steel with low hardenability as the starting material of the second steel material 14, the toughness and the deformability of the portions other than the burring portion 26 can be improved.

(Example 2) A steel with which the Vickers hardness of the leading end portion of the burring portion becomes equal to or more than the Vickers hardness of the first plate-shaped portion is used as the starting material of the first steel material. In this example, the Vickers hardness of the leading end portion of the burring portion is preferably 471 or more, 475 or more, 477 or more, 609 or more, 616 or more, 632 or more, 682 or more, or 684 or more. In addition, the Vickers hardness of the first plate-shaped portion is preferably 463 or more, 465 or more, 468 or more, 602 or more, 603 or more, 625 or more, 676 or more, or 680 or more.

When producing the formed product 10 according to the present embodiment, burring processing and quenching are performed as a series of steps. Here, the burring portion 26 is formed by pushing a pilot hole outward. Therefore, a large strain is applied to the burring portion 26. As a result of the research of the present inventors, it has been found that the hardenability of the burring portion 26 may be decreased by a large strain applied to the burring portion 26. Accordingly, the hardness of the burring portion 26 cannot be increased to a desired hardness by quenching that is performed consecutively to burring processing.

Therefore, in this example, a steel with which the Vickers hardness of the leading end portion of the burring portion 26 becomes equal to or more than the Vickers hardness of the first plate-shaped portion 24 by quenching is used as the starting material of the first steel material 12. Accordingly, the hardness of the burring portion 26 can be sufficiently ensured. Note that, in this example, the requirements in (Example 1) described above may be satisfied, or the requirements in (Example 1) described above may not be satisfied. Specifically, when satisfying the requirements in (Example 1), the starting materials of the first steel material 12 and the second steel material 14 are selected such that the Vickers hardness of the leading end portion of the burring portion 26 and the Vickers hardness of the first plate-shaped portion 24 become equal to or more than the Vickers hardness of the second plate-shaped portion 44, and the Vickers hardness of the leading end portion of the burring portion 26 becomes equal to or more than the Vickers hardness of the first plate-shaped portion 24. In addition, when not satisfying the requirements in (Example 1), the starting materials of the first steel material 12 and the second steel material 14 are selected such that, for example, the Vickers hardness of the leading end portion of the burring portion 26 and the Vickers hardness of the first plate-shaped portion 24 become less than the Vickers hardness of the second plate-shaped portion 44, and the Vickers hardness of the leading end portion of the burring portion 26 becomes equal to or more than the Vickers hardness of the first plate-shaped portion 24. Furthermore, when not satisfying the requirements in (Example 1), the starting materials of the first steel material 12 and the second steel material 14 are selected such that, for example, the Vickers hardness of the leading end portion of the burring portion 26 becomes equal to or more than the Vickers hardness of the second plate-shaped portion 44, and the Vickers hardness of the first plate-shaped portion 24 becomes less than the Vickers hardness of the second plate-shaped portion 44.

(Example 3) In (Example 1) or (Example 2) described above, a steel whose hardenability is decreased by a large strain can be used as the starting material of the second steel material. Specifically, a steel with which the second plate-shaped portion can satisfy the following formula (i) can be used as the starting material of the second steel material.


HV1<HV2  (i)

In the formula (i), HV1 represents the Vickers hardness of a first test specimen in a case where quenching is performed after applying strain to the first test specimen that is cut out from the second plate-shaped portion, and HV2 represents the Vickers hardness of a second test specimen in a case where quenching is performed without applying strain to the second test specimen that is cut out from the second plate-shaped portion. A method of performing quenching on the first test specimen and the second test specimen will be described later. In this example, the Vickers hardness of the second plate-shaped portion is preferably 418 or more, 425 or more, 435 or more, 453 or more, 460 or more, 462 or more, 463 or more, 464 or more, 468 or more, 485 or more, 555 or more, 588 or more, 605 or more, 624 or more, 660 or more, or 661 or more.

Note that, in the formed product 10 according to the present embodiment, since the burring portion 26 is not formed in the second steel material 14, a large strain is not applied to the second steel material 14. Therefore, even when a steel that can satisfy the formula (i), that is, a steel whose hardenability is decreased due to a large strain, is used as the starting material of the second steel material 14, the formed product 10 is not affected by the decrease in the hardenability of the second steel material 14. On the other hand, by selecting the starting material of the first steel material 12 as described in (Example 1) or (Example 2) described above, the hardness of the burring portion 26 can be sufficiently ensured.

Note that the first test specimen and the second test specimen are cut out at positions that are sufficiently distant from the welding seam 16. When the first steel material 12 and the second steel material 14 are welded together by laser welding, the positions that are sufficiently distant from the welding seam 16 are positions 3 mm or more distant from the boundary between the welding seam 16 and the second steel material 14, and when the first steel material 12 and the second steel material 14 are welded together by arc welding, the positions that are sufficiently distant from the welding seam 16 are positions 10 mm or more distant from the boundary.

Hereinafter, the method of performing quenching on the first test specimen and the second test specimen will be described. In the present embodiment, the first test specimen is subjected to hot strain and then immediately quenched. Specifically, first, the first test specimen is heated to 900° C. (however, Ac3+50° C. in the case of a steel material having an Ac3 point exceeding 900° C.), and is held for 60 seconds at the temperature. Then, the first test specimen is cooled to 700° C. (processing temperature) at a cooling rate of 25° C./s, and compression processing is immediately performed on the first test specimen. Then, the first test specimen is immediately cooled to a temperature equal to or below the Mf point at a cooling rate of 50° C./s. Then, the first test specimen is allowed to cool to room temperature.

More specifically, in the present embodiment, an odd number of first test specimens each having a disk shape with a diameter of 8 mm are cut out from the second plate-shaped portion 44, and a cylindrical test piece is prepared by using the first test specimens that have been cut out. FIG. 2 is a schematic diagram illustrating the cylindrical test piece prepared by using the first test specimens.

As illustrated in FIG. 2, a test piece 100 includes a main body portion 50 constituted by an odd number of (three in FIG. 2) first test specimens 52, and a pair of steel dummy materials 60 provided so as to sandwich the main body portion 50. The main body portion 50 is constituted by a minimum number (but an odd number) of first test specimens 52 that form a thickness Ts of 3.0 mm or more. For example, when the thickness of the second plate-shaped portion 44 (the first test specimen 52) is 1.2 mm or more and less than 3.0 mm, the main body portion 50 is constituted by three first test specimens 52. In addition, for example, when the thickness of the second plate-shaped portion 44 (the first test specimen 52) is 3.0 mm or more, the main body portion 50 is constituted by one first test specimen 52. Note that when a scale, a decarburization layer, a metal coating solid solution layer, and the like are formed in the surface layer portion of the second plate-shaped portion 44 (the first test specimen 52), the main body portion 50 is constituted by the first test specimen 52 from which the surface layer portion has been removed by polishing or the like.

Each of the pair of dummy materials 60 has a disc shape with a diameter of 8 mm. Note that the steel type (chemical composition) of the dummy material 60 may be the same as that of the first test specimen 52, or may not be the same as that of the first test specimen 52. However, when the steel type of the dummy material 60 is not the same as that of the first test specimen 52, a steel containing, by mass %, a content of C of 0.1 to 0.5%, and a total content of Cr, Mo, V, W, and Nb of less than 2.0% is used as the starting material of the dummy material 60. The thickness of each of the pair of dummy materials 60 is substantially equal to each other. In the present embodiment, the thickness of the pair of dummy materials 60 is adjusted so that the height of the test piece 100 becomes 12.0 mm. Using a processing formastor testing device, the test piece 100 is subjected to hot compression processing so that the height of the test piece 100 becomes 7.2 mm (the height that is 60% of the original height). The strain rate in the compression processing is set to 5 s−1. Then, quenching is performed on the test piece 100. After the quenching, the first test specimen 52 arranged in the middle portion of the main body portion 50 is cut in the thickness direction along the major axis. In an obtained cut surface, the Vickers hardness is measured at five locations at a t3/4 (t3 is the thickness after the compression processing of the first test specimen 52 at a Vickers hardness measurement position) depth position from an upper surface of the first test specimen 52, and along the direction of the diameter, with the measurement center being set at a position that is ¼ of the diameter (major axis) from the outer circumference, with a test force of 9.8 N (1 kgf) and a pitch of 0.5 mm. The average value of the measured values is used as HV1 of the formula (i).

Quenching is performed on the second test specimen without applying strain. Specifically, similarly to the first test specimen, the second test specimen is heated to 900° C. (however, Ac3+50° C. in the case of a steel material having an Ac3 point exceeding 900° C.), and is held for 60 seconds at the temperature. Then, the second test specimen is cooled at a cooling rate of 25° C./s to 700° C., and at a cooling rate of 50° C./s from 700° C. to a temperature of the Mf point or less. Then, similarly to the first test specimen, the second test specimen that has been cooled to a temperature of the Mf point or less is allowed to cool to room temperature.

Note that, in the present embodiment, as in the case of the first test specimen described above, an odd number of second test specimens each having a disc shape with a diameter of 8 mm are cut out from the second plate-shaped portion 44, and a cylindrical test piece having a height of 12.0 mm is prepared by using the second test specimens that have been cut out. Then, using a processing formastor testing device, heat treatment is performed without processing, according to the thermal history described above. Then, the second test specimen arranged in the middle portion of the test piece (the main body portion) is cut in the thickness direction so as to be equally divided into two pieces. In an obtained cut surface, the Vickers hardness is measured at five locations at a t4/4 (t4 is the thickness of the second test specimen at a Vickers hardness measurement position) depth position from an upper surface of the second test specimen, and along the direction of the diameter, with the measurement center being set at a position that is ¼ of the diameter from the outer circumference, with a test force of 9.8 N (1 kgf) and a pitch of 0.5 mm. The average value of the measured values is used as HV2 of the formula (i).

(Example 4) In (Example 1) or (Example 2) described above, a steel capable of suppressing decrease in the hardenability due to large strain can be used as the starting material of the first steel material. Specifically, a steel with which the first plate-shaped portion can satisfy the following formula (ii) can be used as the starting material of the first steel material.


HV3≥HV4  (ii)

Note that, in the formula (ii), HV3 represents the Vickers hardness of a third test specimen in a case where quenching is performed after applying strain to the third test specimen that is cut out from the first plate-shaped portion, and HV4 represents the Vickers hardness of a fourth test specimen in a case where quenching is performed without applying strain to the fourth test specimen that is cut out from the first plate-shaped portion. In this example, the Vickers hardness of the leading end portion of the burring portion is preferably 471 or more, 475 or more, 477 or more, 609 or more, 616 or more, 632 or more, 682 or more, or 684 or more. In addition, the Vickers hardness of the first plate-shaped portion is preferably 463 or more, 465 or more, 468 or more, 602 or more, 603 or more, 625 or more, 676 or more, or 680 or more.

Note that the third test specimen and the fourth test specimen are cut out at positions that are sufficiently distant from the welding seam 16 and the burring portion 26. Note that the position that is sufficiently distant from the welding seam 16 is a position 3 mm or more distant from the boundary between the welding seam 16 and the first steel material 12 in a case where the first steel material 12 and the second steel material 14 are welded together by laser welding, and is a position 10 mm or more distant from the boundary in a case where the first steel material 12 and the second steel material 14 are welded together by arc welding. In addition, the position that is sufficiently distant from the burring portion 26 is a position that is 10 mm or more distant from the rising position of the burring portion 26.

In addition, in the present embodiment, as in the case of the first test specimen and the second test specimen described above, a test piece is prepared by using an odd number of third test specimens and an odd number of fourth test specimens, and quenching is performed on the prepared test piece. The third test specimen is subjected to hot strain and then is quenched under similar conditions as the first test specimen described above. In addition, the fourth test specimen is quenched without applying strain under similar conditions as the second test specimen described above. HV3 and HV4 of the formula (ii) are measured in a manner similar to that of HV1 and HV2 of the formula (i).

(Example 5) In (Example 1) or (Example 2) described above, the thickness of the leading end portion of the burring portion may be smaller than the thickness of the first plate-shaped portion, and a formed product may satisfy the following formula (iii).


D1>D2  (iii)

In the formula (iii), D1 represents the difference (absolute value) between the thickness of the leading end portion of the burring portion and the thickness of the first plate-shaped portion, and D2 represents the difference (absolute value) between the thickness of the first plate-shaped portion and the thickness of the second plate-shaped portion. In this example, the Vickers hardness of the leading end portion of the burring portion, the Vickers hardness of the first plate-shaped portion, and the Vickers hardness of the second plate-shaped portion are similar to those in (Example 2) to (Example 4) described above, respectively.

That formed product 10 satisfying the formula (iii) means that the difference in thickness between the burring portion 26 and the first plate-shaped portion 24 is larger than the difference in thickness between the first plate-shaped portion 24 and the second plate-shaped portion 44. In other words, it means that the thickness of the first plate-shaped portion 24 and the thickness of the second plate-shaped portion 44 are equivalent. For example, a case is conceivable where a tailored blank is constituted by a first steel sheet and a second steel sheet having equivalent thicknesses, and burring processing is performed on the first steel sheet of the tailored blank. In this case, although the thickness of the leading end portion of the burring portion 26 becomes smaller than the thickness of the first plate-shaped portion 24, the first plate-shaped portion 24 and the second plate-shaped portion 44 have equivalent thicknesses and satisfy the formula (iii). In this manner, even in a case where the thickness of the leading end portion of the burring portion 26 becomes small, the strength of the formed product 10 can be sufficiently ensured by satisfying the requirements in (Example 1) or (Example 2) described above. Note that the thickness of the leading end portion of the burring portion 26 is measured at a position that is 5 mm from the leading end of the burring portion 26 toward the first plate-shaped portion 24.

(Example 6) The starting materials of the first steel material and the second steel material are selected so that the Vickers hardness of the leading end portion of the burring portion becomes smaller than the Vickers hardness of the first plate-shaped portion. In this example, the Vickers hardness of the leading end portion of the burring portion is preferably 382 or more, 433 or more, 442 or more, 455 or more, 479 or more, 534 or more, 536 or more, 542 or more, 571 or more, 578 or more, 582 or more, or 583 or more. In addition, the Vickers hardness of the first plate-shaped portion is preferably 458 or more, 460 or more, 465 or more, 485 or more, 557 or more, 595 or more, 609 or more, 610 or more, 625 or more, 660 or more, 662 or more, or 663 or more.

When utilizing the formed product 10 as an automobile part, it is conceivable that another part is attached to the burring portion 26. In this case, it is necessary to perform processing, such as trimming and piercing, on the burring portion 26. Therefore, the starting material of the first steel material 12 is selected so that the Vickers hardness of the leading end portion of the burring portion 26 becomes smaller than the Vickers hardness of the first plate-shaped portion 24. That is, a steel with which the hardness of the leading end portion of the burring portion 26 becomes lower than the hardness of the first plate-shaped portion 24 after quenching is used as the starting material of the first steel material 12. In this case, processing (trimming, piercing, and the like) of the burring portion 26 becomes easy. Note that, in this example, the requirements in (Example 1) described above may be satisfied, or the requirements in (Example 1) described above may not be satisfied. When satisfying the requirements in (Example 1), the starting materials of the first steel material 12 and the second steel material 14 are selected such that the Vickers hardness of the leading end portion of the burring portion 26 and the Vickers hardness of the first plate-shaped portion 24 become equal to or more than the Vickers hardness of the second plate-shaped portion 44, and the Vickers hardness of the leading end portion of the burring portion 26 becomes less than the Vickers hardness of the first plate-shaped portion 24. In addition, when not satisfying the requirements in (Example 1), the starting materials of the first steel material 12 and the second steel material 14 are selected such that, for example, the Vickers hardness of the leading end portion of the burring portion 26 and the Vickers hardness of the first plate-shaped portion 24 become less than the Vickers hardness of the second plate-shaped portion 44, and the Vickers hardness of the leading end portion of the burring portion 26 becomes less than the Vickers hardness of the first plate-shaped portion 24. Furthermore, when not satisfying the requirements in (Example 1), the starting materials of the first steel material 12 and the second steel material 14 are selected such that, for example, the Vickers hardness of the first plate-shaped portion 24 becomes equal to or more than the Vickers hardness of the second plate-shaped portion 44, and the Vickers hardness of the leading end portion of the burring portion 26 becomes less than the Vickers hardness of the second plate-shaped portion 44.

When satisfying the requirements in (Example 1) described above, the starting materials of the first steel material 12 and the second steel material 14 are selected such that, for example, the carbon content of the first steel material 12 becomes greater than the carbon content of the second steel material 14. Specifically, for example, the steel A described above can be used as the starting material of the first steel material 12, and the steel B described above can be used as the starting material of the second steel material 14. In addition, when satisfying the requirements in (Example 1) described above, for example, the steel described in (Example 3) described above may be used as the starting material of the second steel material 14.

In addition, when not satisfying the requirements in (Example 1) described above, the starting materials of the first steel material 12 and the second steel material 14 are selected such that, for example, the carbon content of the first steel material 12 becomes less than the carbon content of the second steel material 14. Specifically, for example, the steel B described above can be used as the starting material of the first steel material 12, and the steel A described above can be used as the starting material of the second steel material 14. In this case, the hydrogen embrittlement resistance of the burring portion 26 can be improved, while ensuring the strength and the wear resistance of the formed product 10.

In addition, when not satisfying the requirements in (Example 1) described above, both the first steel material 12 and the second steel material 14 may satisfy the conditions of the steel A described above, or both the first steel material 12 and the second steel material 14 may satisfy the conditions of the steel B described above. In these cases, for example, the carbon contents of the starting materials of the first steel material 12 and the second steel material 14 may be equivalent, and a steel with low hardenability may be used as the starting material of the first steel material 12, and a steel with high hardenability may be used as the starting material of the second steel material 14. Specifically, a steel having a Ceq of less than 0.65 can be used as the starting material of the first steel material 12, and a steel having a Ceq of 0.65 or more can be used as the starting material of the second steel material 14.

In addition, when not satisfying the requirements in (Example 1) described above, a steel with a higher carbon content and lower hardenability than the starting material of the second steel material 14 may be used as the starting material of the first steel material 12, and a steel with high hardenability may be used as the starting material of the second steel material 14. Specifically, a steel having a Ceq of less than 0.65 can be used as the starting material of the first steel material 12, and a steel having a Ceq of 0.65 or more can be used as the starting material of the second steel material 14. In this case, the workability of the burring portion 26 can be improved, and the deformability and hydrogen embrittlement resistance of the portions other than the burring portion 26 can be improved.

(Example 7) The first steel sheet and the second steel sheet are selected so that the thickness of the first plate-shaped portion becomes greater than the thickness of the second plate-shaped portion. In this example, the Vickers hardness of the leading end portion of the burring portion, the Vickers hardness of the first plate-shaped portion, and the Vickers hardness of the second plate-shaped portion are similar to those in (Example 2) to (Example 6) described above, respectively.

In this example, a steel sheet with a large thickness can be utilized as the first steel sheet that serves as the first steel material 12 in the formed product 10, and a steel sheet with a small thickness can be utilized as the second steel sheet that serves as the second steel material 14 in the formed product 10. In this case, the corrosion resistance life and fatigue properties of the burring portion 26 and its periphery can be improved, while suppressing an increase in the weight of the formed product 10. In addition, by utilizing a steel sheet with a large thickness as the first steel sheet, when producing the formed product 10, it is possible to prevent the temperature of the first steel material 12 (the first steel sheet) from decreasing after a tailored blank is carried out from a furnace (after the second step) and before burring processing (the fourth step) is performed. Accordingly, the appearance of a ferrite layer can be suppressed, and softening of the burring portion 26 can be suppressed. In addition, when cooling the burring portion 26 with a die set, it is possible to prevent the cooling rate of the burring portion 26 from becoming excessively high. Accordingly, the toughness of the burring portion 26 can be prevented from decreasing.

(Example 8) The first steel sheet and the second steel sheet are selected so that the thickness of the first plate-shaped portion becomes smaller than the thickness of the second plate-shaped portion. In this example, the Vickers hardness of the leading end portion of the burring portion, the Vickers hardness of the first plate-shaped portion, and the Vickers hardness of the second plate-shaped portion are similar to those in (Example 2) to (Example 7) described above, respectively.

In this example, a steel sheet with a small thickness can be utilized as the first steel sheet that serves as the first steel material 12 in the formed product 10, and a steel sheet with a large thickness can be utilized as the second steel sheet that serves as the second steel material 14 in the formed product 10. In this case, by utilizing a steel sheet with a small thickness as the first steel sheet, a sufficient cooling rate can be maintained when cooling the burring portion 26 with a die set. Accordingly, the appearance of a ferrite layer can be suppressed, and softening of the burring portion 26 can be suppressed. In addition, the corrosion resistance and fatigue properties of the portions other than the burring portion 26 can be improved by increasing the thickness of the second plate-shaped portion 44.

(Example 9) When producing a tailored blank, a metal coated steel sheet is utilized for at least one of the first steel sheet (first steel material) and the second steel sheet (second steel material). In this example, the Vickers hardness of the leading end portion of the burring portion, the Vickers hardness of the first plate-shaped portion, and the Vickers hardness of the second plate-shaped portion are similar to those in (Example 2) to (Example 8) described above, respectively.

According to this example, the corrosion resistance of required portions of the formed product 10 can be improved. For example, a steel sheet including a Zn-based coating layer can be utilized as the second steel sheet. In this case, the first steel sheet may not include a metal coating layer, or may include an Al—Si based coating layer. In addition, for example, a steel sheet including an Al—Si based coating layer may be utilized as the second steel sheet. In this case, the first steel sheet may not include a metal coating layer, or may include a Zn-based coating layer.

Hereinafter, although the present invention will be described more specifically with examples, the present invention is not limited to these examples.

Example

First steel sheets and second steel sheets having the thicknesses shown in Table 2 were prepared by using steels having the chemical composition of steel types a to s shown in Table 1 as starting materials. Tailored blanks were prepared by laser welding the obtained first steel sheets and second steel sheets together (the first step). Note that, as shown in Table 2, surfaces of the first steel sheets of Test numbers 1 and 2 and the second steel sheets of Test numbers 12 and 20 were subjected to alloyed hot-dip galvanizing before laser welding. In addition, in the present example, a square steel sheet of 140 mm×140 mm was used as each of the first steel sheet and the second steel sheet to make a tailored blank of 140 mm×280 mm. In order to perform burring processing, which will be described later, a pilot hole with a diameter of 20 mm was formed in a center portion of the first steel sheet before laser welding.

TABLE 1 Steel Chemical Composition (Mass %, Balance: Fe and Impurities) Ac3 Mf Type C Si Mn P S N Al Cr Nb Ti Mo B Ni V Ceq (° C.) (° C.) a 0.15 0.10 2.11 0.007 0.0011 0.0029 0.029 0.210 0.002 — — 0.0020 — — 0.55 856 265 b 0.14 0.11 1.97 0.008 0.0014 0.0024 0.031 0.180 0.003 0.022 — 0.0015 — — 0.51 861 275 c 0.21 0.22 1.23 0.014 0.0017 0.0032 0.033 0.190 0.003 0.021 0.007 0.0015 — — 0.46 862 273 d 0.20 0.05 1.15 0.009 0.0012 0.0029 0.027 0.180 0.002 0.019 0.005 0.0014 — — 0.43 860 282 e 0.25 0.29 1.32 0.008 0.0009 0.0024 0.034 0.180 0.003 0.024 0.035 0.0019 — — 0.53 866 253 f 0.22 1.01 2.39 0.010 0.0016 0.0020 0.035 0.370 0.002 0.024 — 0.0014 0.02 — 0.73 866 216 g 0.21 0.10 1.92 0.009 0.0007 0.0033 0.038 0.350 0.002 0.019 — — — — 0.60 863 245 h 0.24 0.16 2.07 0.005 0.0008 0.0035 0.029 0.390 0.003 0.026 0.025 — 0.02 — 0.68 869 225 i 0.32 0.22 1.71 0.009 0.0007 0.0033 0.038 0.180 0.030 0.022 0.007 0.0017 — — 0.65 874 210 j 0.27 0.07 1.22 0.008 0.0008 0.0033 0.028 0.180 0.027 0.019 — 0.0015 — — 0.51 869 250 k 0.32 0.29 1.33 0.009 0.0009 0.0035 0.033 0.220 0.031 0.018 0.023 0.0020 — — 0.60 874 223 l 0.35 0.23 1.31 0.005 0.0008 0.0035 0.029 0.190 0.045 0.025 0.012 0.0019 — — 0.62 880 213 m 0.38 0.29 1.35 0.008 0.0009 0.0038 0.033 0.200 0.033 0.021 0.024 0.0020 — 0.17 0.68 876 199 n 0.33 0.15 1.32 0.006 0.0007 0.0037 0.031 0.019 0.042 0.022 0.013 0.0021 — — 0.56 876 225 o 0.32 0.21 1.26 0.008 0.0005 0.0035 0.028 0.017 0.031 0.019 — 0.0018 — — 0.54 870 231 p 0.36 0.29 1.34 0.012 0.0009 0.0035 0.031 0.021 0.033 0.022 0.023 0.0022 — — 0.61 875 211 q 0.40 0.19 0.78 0.010 0.0003 0.0038 0.028 0.190 0.076 0.020 — 0.0014 — — 0.58 887 213 r 0.46 0.39 0.41 0.011 0.0005 0.0038 0.039 0.280 0.018 0.021 0.211 0.0021 — — 0.65 887 198 s 0.47 0.31 0.39 0.011 0.0005 0.0038 0.039 0.390 0.018 0.026 0.100 0.0020 — — 0.65 879 194

TABLE 2 First Steel Sheet Second Steel Sheet Test Steel Thickness Ceq(A) C(A) Metal Steel Thickness Ceq(B) C(B) Metal Ceq(A) − Ceq(B) C(A) − C(B) Number Type (mm) (%) (%) Coating Type (mm) (%) (%) Coating (%) (%) 1 p 2.59 0.61 0.36 GA e 2.60 0.53 0.25 — 0.08 0.11 2 p 2.59 0.61 0.36 GA g 2.60 0.60 0.21 — 0.01 0.15 3 q 2.62 0.58 0.40 — j 2.59 0.51 0.27 — 0.07 0.13 4 f 2.58 0.73 0.22 — c 2.61 0.46 0.21 — 0.27 0.01 5 f 2.61 0.73 0.22 — d 2.31 0.43 0.20 — 0.30 0.02 6 g 2.60 0.60 0.21 — d 2.30 0.43 0.20 — 0.17 0.01 7 r 2.59 0.65 0.46 — q 2.59 0.58 0.40 — 0.07 0.06 8 c 2.61 0.46 0.21 — a 2.31 0.55 0.15 — −0.09 0.06 9 e 2.60 0.53 0.25 — b 2.61 0.51 0.14 — 0.02 0.11 10 q 2.27 0.58 0.40 — j 2.60 0.51 0.27 — 0.07 0.13 11 h 2.31 0.68 0.24 — f 2.61 0.73 0.22 — −0.05 0.02 12 m 2.29 0.68 0.38 — o 2.58 0.54 0.32 GA 0.14 0.06 13 s 2.60 0.65 0.47 — q 2.60 0.58 0.40 — 0.07 0.07 14 q 2.58 0.58 0.40 — f 2.59 0.73 0.22 — −0.15 0.18 15 l 2.59 0.62 0.35 — h 2.59 0.68 0.24 — −0.06 0.11 16 i 2.62 0.65 0.32 — n 2.58 0.56 0.33 — 0.09 −0.01 17 k 2.61 0.60 0.32 — a 2.61 0.55 0.15 — 0.05 0.17 18 d 2.60 0.43 0.20 — i 2.60 0.65 0.32 — −0.22 −0.12 19 j 2.62 0.51 0.27 — k 2.62 0.60 0.32 — −0.09 −0.05 20 h 2.59 0.68 0.24 — p 2.58 0.61 0.36 GA 0.07 −0.12 21 i 2.62 0.65 0.32 — c 2.61 0.46 0.21 — 0.19 0.11 22 l 2.59 0.62 0.35 — c 2.61 0.46 0.21 — 0.16 0.14 *In the table, Ceq (A) represents the carbon equivalent of the first steel sheet, Ceq (B) represents the carbon equivalent of the second steel sheet, C (A) represents the carbon content of the first steel sheet, and C (B) represents the carbon content of the second steel sheet.

Heating (the second step), press forming (the third step), burring processing (the fourth step), and quenching (the fifth step) were performed on the obtained tailored blanks to obtain formed products similar in their shapes to the formed product 10 illustrated in FIG. 1. Note that, in the present example, heating of the tailored blanks in the second step was performed by using a heating furnace (gas furnace). The furnace temperature setting was set to 900 to 930° C., and the in-furnace time was set to 4 to 7 minutes depending on the thickness. In the present example, the third step and the fourth step were started at the same time, and then the fifth step was performed. Table 3 shows the heating temperature (steel sheet surface temperature) in the second step, the surface temperature of the tailored blank at the start of processing in the third step and the fourth step, the average cooling rate in the fifth step after the processing in the third step and the fourth step (the surface temperature of the formed product), and the surface temperature of the formed product at the time of removal from a die set. Note that, in the present example, a burring portion with an inner diameter of 50 mm and a height of 16 mm to 23 mm was formed by burring processing in the fourth step.

TABLE 3 Temperature at Ac3 (° C.) Mf (° C.) Heating Processing Start Average Cooling Rate Removal from Test First Steel Second Steel First Steel Second Steel Temperature Temperature after Processing Die Set Number Sheet Sheet Sheet Sheet (° C.) (° C.) (° C./s) (° C.) 1 875 866 211 253 902 712 54 <70 2 875 863 211 245 904 689 71 <70 3 887 869 213 250 901 703 63 <70 4 866 862 216 273 903 742 67 <70 5 866 860 216 282 900 731 76 <70 6 863 860 245 282 899 704 72 <70 7 887 887 198 213 903 711 62 <70 8 862 856 273 265 902 704 54 <70 9 866 861 253 275 923 712 48 <70 10 887 869 213 250 925 688 67 <70 11 869 866 225 216 921 724 71 <70 12 876 870 199 231 922 723 66 <70 13 879 887 194 213 903 718 74 <70 14 887 866 213 216 904 692 62 <70 15 880 869 213 225 902 708 58 <70 16 874 876 210 225 913 712 71 <70 17 874 856 223 265 922 688 66 <70 18 860 874 282 210 898 701 65 <70 19 869 874 250 223 901 704 74 <70 20 869 875 225 211 904 716 59 <70 21 874 862 210 273 904 706 69 <70 22 880 862 213 273 913 694 64 <70

The Vickers hardness and thickness of each portion were measured for the formed products of Test numbers 1 to 22 obtained as described above. The method of measuring the Vickers hardness and thickness of each portion is as described above. In addition, the Vickers hardness was also measured for the first test specimen (quenched with strain), the second test specimen (quenched without strain), the third test specimen (quenched with strain), and the fourth test specimen (quenched without strain) that have been described above. Table 4 shows the measurement results. Note that the Vickers hardness of the first test specimen, the second test specimen, the third test specimen, and the fourth test specimen was measured in accordance with the method using the processing formastor testing device described above. Note that Table 4 also shows the corresponding relationships between the respective formed products of Test numbers 1 to 22 and (Example 1) to (Example 9) described above.

TABLE 4 Vickers Hardness (HV1) First Steel Material Leading End Second Steel Material Portion of First Plate- Third Test Fourth Test Second Plate- First Test Second Test Hardness Difference Test Burring Portion Shaped Portion Specimen Specimen Shaped Portion Specimen Specimen HV(HS1) − HV(LS1) − Number (HS1) HV (LS1) HV3 HV4 HV (LS2) HV1 HV2 HV(LS2) HV(LS2) 1 534 625 527 627 485 422 488 49 140 2 536 625 519 631 460 431 474 76 165 3 582 662 564 659 555 463 549 27 107 4 475 465 468 466 463 418 472 12 2 5 471 463 462 458 462 398 456 9 1 6 455 460 427 461 453 403 448 2 7 7 684 680 681 676 660 588 655 24 20 8 433 465 397 459 425 376 419 8 40 9 442 485 421 477 435 383 425 7 50 10 582 660 577 654 555 492 563 27 105 11 477 468 467 466 466 461 458 11 2 12 632 625 628 622 588 529 584 44 37 13 682 676 673 669 661 577 654 21 15 14 571 663 544 660 468 463 457 103 195 15 583 610 573 614 475 477 471 108 135 16 616 602 609 603 605 553 605 11 −3 17 542 595 546 601 418 378 411 124 177 18 382 458 387 452 611 612 608 −229 −153 19 479 557 463 564 608 509 606 −129 −51 20 475 468 472 466 624 528 619 −149 −156 21 609 603 608 599 468 421 466 141 135 22 578 609 575 611 464 417 468 114 145 Thickness (mm) First Steel Material Second Steel Corresponding Vickers Hardness (HV1) Leading End Material Thickness Difference Relationships to Hardness Difference Portion of First Plate- Second Plate- D1 D2 Examples 1 Test HV(HS1) − Burring Portion Shaped Portion Shaped Portion | t(HS1) − | t(LS1) − to 9 in Number HV(LS1) t (HS1) t (LS1) t (LS2) t(LS1) | t(LS2) | Description 1 −91 1.62 2.61 2.61 0.99 0.00 (Example) 1, 1a, 3, 5, 6, 9 2 −89 1.61 2.59 2.60 0.98 0.01 (Example) 1, 1a, 3, 5, 6, 8, 9 3 −80 1.63 2.60 2.59 0.97 0.01 (Example) 1, 1a, 3, 5, 6, 7 4 10 1.67 2.60 2.60 0.93 0.00 (Example) 1, 1a, 1b, 2, 3, 4, 5 5 8 1.59 2.63 2.29 1.04 0.34 (Example) 1, 1a, 1b, 2, 3, 4, 5, 7 6 −5 1.71 2.61 2.31 0.90 0.30 (Example) 1, 1a, 3, 5, 6, 7 7 4 1.64 2.59 2.60 0.95 0.01 (Example) 1, 1a, 1b, 2, 3, 4, 5, 8 8 −32 1.66 2.59 2.29 0.93 0.30 (Example) 1, 1a, 3, 5, 6, 7 9 −43 1.63 2.60 2.61 0.97 0.01 (Example) 1, 1a, 3, 5, 6, 8 10 −78 1.41 2.29 2.61 0.88 0.32 (Example) 1, 1a, 3, 5, 6, 8 11 9 1.47 2.29 2.59 0.82 0.30 (Example) 1, 1a, 2, 4, 5, 8 12 7 1.48 2.30 2.60 0.82 0.30 (Example) 1, 1a, 2, 3, 4, 5, 8, 9 13 6 1.73 2.61 2.60 0.88 0.01 (Example) 1, 1a, 1b, 2, 3, 4, 5, 7 14 −92 1.61 2.59 2.61 0.98 0.02 (Example) 1, 1a, 5, 6, 8 15 −27 1.69 2.59 2.59 0.90 0.00 (Exam ple) 1, 1a, 5, 6 16 14 1.64 2.60 2.60 0.96 0.00 (Example) 2, 3, 4, 5 17 −53 1.66 2.61 2.59 0.95 0.02 (Example) 1, 1a, 3, 5, 6, 7 18 −76 1.66 2.61 2.60 0.95 0.01 (Example) 6, 7 19 −78 1.61 2.59 2.61 0.98 0.02 (Example) 6, 8 20 7 1.64 2.60 2.61 0.96 0.01 (Example) 2, 3, 4, 5, 8, 9 21 6 1.65 2.60 2.60 0.95 0.00 (Example) 1, 1a, 1b, 2, 3, 4, 5 22 −31 1.63 2.59 2.60 0.96 0.01 (Example) 1, 1a, 3, 5, 6, 8

As shown in Table 4, the formed products corresponding to all of (Example 1) to (Example 9) could be prepared. From this result, it was found that, according to the present invention, a hot stamped product with high design freedom can be obtained that allows adjustment of the mechanical properties of each portion according to needs.

INDUSTRIAL APPLICABILITY

According to the present invention, a hot stamped product with high design freedom can be obtained.

REFERENCE SIGNS LIST

    • 10 hot-stamped product
    • 12 first steel material
    • 14 second steel material
    • 16 welding seam
    • 20, 22 first vertical wall portion
    • 24 first plate-shaped portion
    • 26 burring portion
    • 40, 42 second vertical wall portion
    • 44 second plate-shaped portion

Claims

1. A hot-stamped product comprising:

a first steel material that includes a first plate-shaped portion and a burring portion rising from the first plate-shaped portion;
a second steel material that includes a second plate-shaped portion provided so as to be aligned with the first plate-shaped portion in a direction perpendicular to a thickness direction of the first plate-shaped portion; and
a welding seam that joins an edge of the first plate-shaped portion and an edge of the second plate-shaped portion.

2. The hot-stamped product according to claim 1, wherein a Vickers hardness of a leading end portion of the burring portion and a Vickers hardness of the first plate-shaped portion are equal to or more than a Vickers hardness of the second plate-shaped portion.

3. The hot-stamped product according to claim 2, wherein the Vickers hardness of the leading end portion of the burring portion is equal to or more than the Vickers hardness of the first plate-shaped portion.

4. The hot-stamped product according to claim 2,

wherein a first test specimen and a second test specimen are cut out from the second plate-shaped portion, and
a Vickers hardness HV1 of the first test specimen in a case where quenching is performed after applying strain to the first test specimen, and a Vickers hardness HV2 of the second test specimen in a case where quenching is performed without applying strain to the second test specimen satisfy the following formula (i): HV1<HV2  (i).

5. The hot-stamped product according to claim 2,

wherein a third test specimen and a fourth test specimen are cut out from the first plate-shaped portion, and
a Vickers hardness HV3 of the third test specimen in a case where quenching is performed after applying strain to the third test specimen, and a Vickers hardness HV4 of the fourth test specimen in a case where quenching is performed without applying strain to the fourth test specimen satisfy the following formula (ii): HV3≥HV4  (ii).

6. The hot-stamped product according to claim 2,

wherein a thickness of the leading end portion of the burring portion is smaller than a thickness of the first plate-shaped portion, and
a difference D1 between the thickness of the leading end portion of the burring portion and the thickness of the first plate-shaped portion, and a difference D2 between the thickness of the first plate-shaped portion and a thickness of the second plate-shaped portion satisfy the following formula (iii): D1>D2  (iii).

7. The hot-stamped product according to claim 1, wherein a Vickers hardness of a leading end portion of the burring portion is smaller than a Vickers hardness of the first plate-shaped portion.

8. A method of producing a hot-stamped product according to claim 1, the method comprising:

a step of obtaining a tailored blank by welding a first steel sheet and a second steel sheet;
a step of heating the tailored blank;
a step of performing press forming on the tailored blank;
a step of performing burring processing on the first steel sheet of the heated tailored blank; and
a step of performing quenching on the heated tailored blank.

9. The method of producing the hot-stamped product according to claim 8, wherein, in the step of obtaining the tailored blank, a metal coated steel sheet is used for at least one of the first steel sheet and the second steel sheet.

10. The hot-stamped product according to claim 3,

wherein a first test specimen and a second test specimen are cut out from the second plate-shaped portion, and
a Vickers hardness HV1 of the first test specimen in a case where quenching is performed after applying strain to the first test specimen, and a Vickers hardness HV2 of the second test specimen in a case where quenching is performed without applying strain to the second test specimen satisfy the following formula (i): HV1<HV2  (i).

11. The hot-stamped product according to claim 3,

wherein a third test specimen and a fourth test specimen are cut out from the first plate-shaped portion, and
a Vickers hardness HV3 of the third test specimen in a case where quenching is performed after applying strain to the third test specimen, and a Vickers hardness HV4 of the fourth test specimen in a case where quenching is performed without applying strain to the fourth test specimen satisfy the following formula (ii): HV3≥HV4  (ii).

12. The hot-stamped product according to claim 3,

wherein a thickness of the leading end portion of the burring portion is smaller than a thickness of the first plate-shaped portion, and
a difference D1 between the thickness of the leading end portion of the burring portion and the thickness of the first plate-shaped portion, and a difference D2 between the thickness of the first plate-shaped portion and a thickness of the second plate-shaped portion satisfy the following formula (iii): D1>D2  (iii).

13. The hot-stamped product according to claim 2, wherein a Vickers hardness of a leading end portion of the burring portion is smaller than a Vickers hardness of the first plate-shaped portion.

Patent History
Publication number: 20260249343
Type: Application
Filed: Mar 13, 2024
Publication Date: Aug 27, 2026
Applicant: NIPPON STEEL CORPORATION (Tokyo)
Inventors: Hitomi NISHIBATA (Tokyo), Keitaro MORI (Tokyo), Toshiya SUZUKI (Tokyo)
Application Number: 19/165,532
Classifications
International Classification: B21D 35/00 (20060101); B21D 22/02 (20060101); B21D 39/02 (20060101);