阅读说明：本技术 热冲压用重叠坯料、重叠热冲压成型体的制造方法以及重叠热冲压成型体 (Overlapped blank for hot stamping, method for producing overlapped hot stamped product, and overlapped hot stamped product ) 是由 藤田宗士 铃木优贵 布田雅裕 真木纯 入川秀昭 中田匡浩 于 2019-04-05 设计创作，主要内容包括：本发明解决重叠部与单张部之间的升温速度差的相关问题,并进一步提高热冲压后的镀层的耐腐蚀性。一种热冲压用重叠坯料,其具备：第一钢板,以及至少一片介由焊接点与第一钢板表面连接的比所述第一钢板面积小的第二钢板；第一钢板为在该第一钢板的两面具有铝系镀层的镀覆钢板,且第二钢板为在该第二钢板的两面具有铝系镀层的镀覆钢板,第一钢板中的铝系镀层的附着量以两面上的平均附着量计为W1(g/m<Sup>2</Sup>),在第二钢板中不与第一钢板接触一侧的表面处的铝系镀层的附着量为W2(g/m<Sup>2</Sup>),W1和W2均在20g/m<Sup>2</Sup>以上且120g/m<Sup>2</Sup>以下的范围内,且满足式(1)和式(2)的关系。(The present invention solves the problem associated with the difference in temperature rise rate between the overlapped portion and the single-piece portion, and further improves the corrosion resistance of the plated layer after hot stamping. A blank for hot stamping comprising: a first steel plate, and at least one second steel plate connected to the surface of the first steel plate via a weld point and having an area smaller than that of the first steel plate; the first steel sheet is a plated steel having aluminum plating layers on both surfaces of the first steel sheeta plate, wherein the second steel plate is a plated steel plate having aluminum-based plating layers on both sides of the second steel plate, and the amount of aluminum-based plating layers adhered to the first steel plate is W1 (g/m) in terms of the average amount of adhesion on both sides 2 ) The amount of aluminum plating deposited on the surface of the second steel sheet not in contact with the first steel sheet was W2 (g/m) 2 ) W1 and W2 are both at 20g/m 2 Above and 120g/m 2 In the following range, and satisfies the relationship between formula (1) and formula (2).)

1. A blank for hot stamping comprising: a first steel plate, and,

At least one second steel plate connected to the surface of the first steel plate via a weld point and having an area smaller than that of the first steel plate;

The first steel sheet is a plated steel sheet having aluminum plating layers on both sides of the first steel sheet, and the second steel sheet is a plated steel sheet having aluminum plating layers on both sides of the second steel sheet,

The amount of the aluminum plating layer adhered to the first steel sheet was W1 (g/m) in terms of the average amount of the aluminum plating layer adhered to both surfaces²)，

The amount of the aluminum plating deposited on the surface of the second steel sheet on the side not in contact with the first steel sheet was W2 (g/m)²)，

The W1 and the W2 are both at 20g/m²Above and 120g/m²In the following range, and satisfies the relationship of the following formulae (1) and (2),

More than or equal to 30 (W1-W2) less than or equal to 100 formula (1)

(W1/W2)²X (t1/t2) ≥ 1.5 formula (2)

In the formula (2), t1(mm) represents the plate thickness of the first steel plate, and t2(mm) represents the plate thickness of the second steel plate.

2. The overlapped blank for hot stamping according to claim 1, wherein,

The welding is spot welding,

The welding spot density of the spot welding is 1 spot/200 cm²The above.

3. The overlapped blank for hot stamping according to claim 2, wherein the first steel plate has a portion that becomes a flange portion constituted only by the first steel plate after being subjected to hot stamping forming,

At least a part of the overlapping portion of the first steel plate and the second steel plate has a portion that becomes a bent portion after being subjected to hot press forming,

At least one spot weld of the spot welding exists in a portion which becomes a bent portion after hot press forming.

4. The superimposed blank for hot stamping according to any one of claims 1 to 3, wherein a plate thickness t1(mm) of the first steel plate and a plate thickness t2(mm) of the second steel plate satisfy the following formula (3),

(t2/t1) is less than or equal to 2.0 and formula (3).

5. The superimposed blank for hot stamping according to any one of claims 1 to 4, wherein the aluminum-based plating layer of each of the first steel sheet and the second steel sheet has a two-layer structure of an aluminum layer and an aluminum-iron-based alloy layer in order from the surface toward the base steel sheet, and a thickness d1(μm) of the aluminum-iron-based alloy layer of the first steel sheet and a thickness d2(μm) of the aluminum-iron-based alloy layer of the second steel sheet satisfy the following formula (4),

2 to 10 (d2-d1) as shown in the formula (4).

6. the superimposed blank for hot stamping according to any one of claims 1 to 5, wherein the second steel sheet further has a carbon-based coating film having an emissivity of 0.7 or more on the surface of the aluminum-based plating layer on the side not in contact with the first steel sheet.

7. The superimposed blank for hot stamping according to any one of claims 1 to 6, wherein the aluminum-based plating layer on the surface of the second steel sheet further comprises ZnO or TiO₂Is composed of at least one of the above components and has an adhesion amount per one surface of0.2g/m²The above film.

8. A method for producing a stacked hot-stamped product, wherein, in hot-stamping a stacked blank for hot stamping comprising a first steel plate and at least one second steel plate having a smaller area than the first steel plate and connected to the surface of the first steel plate via a weld, and shaping the heated stacked blank, a bent portion obtained by bending is provided at least in a part of a portion where the first steel plate and the second steel plate are stacked,

The first steel sheet is a plated steel sheet having aluminum plating layers on both sides of the first steel sheet, and the second steel sheet is a plated steel sheet having aluminum plating layers on both sides of the second steel sheet,

The amount of the aluminum plating layer adhered to the first steel sheet was W1 (g/m) in terms of the average amount of the aluminum plating layer adhered to both surfaces²)，

The amount of the aluminum plating deposited on the surface of the second steel sheet on the side not in contact with the first steel sheet was W2 (g/m)²)，

The W1 and the W2 are both at 20g/m²Above and 120g/m²In the following range, and satisfies the relationship of the following formulae (1) and (2),

More than or equal to 30 (W1-W2) less than or equal to 100 formula (1)

(W1/W2)²X (t1/t2) ≥ 1.5 formula (2)

In the formula (2), t1(mm) represents the plate thickness of the first steel plate, and t2(mm) represents the plate thickness of the second steel plate.

9. The method of producing a superimposed hot stamped steel according to claim 8, wherein,

The welding is spot welding,

The welding spot density of the spot welding is 1 spot/200 cm²The above.

10. The method of manufacturing a superimposed hot stamped shape according to claim 9, wherein at least one spot weld of the spot welding is present in a portion which becomes the bent portion after hot stamping.

11. The method for producing a stacked hot stamped steel according to any one of claims 8 to 10, wherein a plate thickness t1(mm) of the first steel plate and a plate thickness t2(mm) of the second steel plate satisfy the relationship of the following formula (3),

(t2/t1) is less than or equal to 2.0 and formula (3).

12. The method for producing a superimposed hot stamped steel according to any one of claims 8 to 11, wherein the aluminum-based plating layer of each of the first steel sheet and the second steel sheet has a two-layer structure of an aluminum layer and an aluminum-iron-based alloy layer in that order from the surface toward the base steel sheet, and the thickness d1(μm) of the aluminum-iron-based alloy layer of the first steel sheet and the thickness d2(μm) of the aluminum-iron-based alloy layer of the second steel sheet satisfy the following formula (4),

2 to 10 (d2-d1) as shown in the formula (4).

13. the method for producing a stacked hot stamped steel according to any one of claims 8 to 12, wherein the second steel sheet further has a carbon-based coating film having an emissivity of 0.7 or more on the surface of the aluminum-based plating layer on the side not in contact with the first steel sheet.

14. The method for producing a stacked hot stamp-formed body according to any one of claims 8 to 13, wherein the aluminum plating layer on the surface of the second steel sheet further has ZnO or TiO on the surface thereof₂At least one of the above components and an amount of the adhesive to one surface of the substrate is 0.2g/m²The above film.

15. A stacked hot stamp-formed body comprising:

A first steel plate having a plate thickness of T1(mm), and,

At least one second steel plate connected to the surface of the first steel plate via a weld, having a smaller area than the first steel plate and a plate thickness of T2 (mm);

The first steel sheet is a plated steel sheet having aluminum plating layers with an average plating thickness of K1 (mum) on both sides of the first steel sheet,

The second steel sheet is a plated steel sheet having an aluminum-based plating layer with a plating thickness of K2(μm) on the surface on the side not in contact with the first steel sheet,

The overlapped hot stamp-molded article satisfies the relationship between the following formula (11) and formula (12),

33 type (11) is more than or equal to 10 (K1-K2)

(K1/K2)²X (T1/T2) ≥ 1.5 formula (12).

Technical Field

The present invention relates to a superimposed blank for hot stamping, a method for producing a superimposed hot stamped product, and a superimposed hot stamped product.

Background

In recent years, steel sheets having both high strength and high formability have been desired for use as steel sheets for automobiles, and as one of steel sheets having both high strength and high formability, trip (transformation Induced plasticity) steel using martensitic transformation of retained austenite is used. The TRIP steel can be used to produce a high-strength steel sheet having a strength of about 1000MPa with excellent formability. However, the technique using TRIP steel has a problem that it is difficult to ensure formability of ultra-high strength steel having higher strength (for example, 1500MPa or more), and shape retention after forming is poor, and dimensional accuracy of a formed product is poor.

As a method of forming at around room temperature (so-called cold pressing method) as described above, hot stamping (also referred to as hot press molding, hot pressing, die quenching, press quenching, and the like) has recently attracted attention. This hot stamping is a manufacturing method of a part in which a steel sheet is heated to Ac3 point or more (for example, 800 ℃ or more) and austenitized, then hot-pressed immediately to ensure formability, and quenched to Ms point or less (for example, 400 ℃ or less) by a die during holding at the bottom dead center to martensite the material and quench, thereby obtaining a desired high-strength material after pressing. According to the method, an automobile part having excellent shape retention after molding can be obtained.

On the other hand, various press-molded articles used for parts constituting an automobile body are required to have various improved performances and characteristics from the viewpoints of static strength, dynamic strength, collision safety, weight reduction, and the like. For example, in automobile parts such as an a-pillar reinforcement, a B-pillar reinforcement, a bumper reinforcement, a tunnel reinforcement, a side sill reinforcement, a roof reinforcement, or a floor cross member, it is required that only a specific portion of each automobile part has a higher impact resistance than general portions other than the specific portion.

Therefore, in the year around 2007, a method of manufacturing a lap hot stamped body by overlapping and welding a plurality of steel sheets only in a portion corresponding to a specific portion to be reinforced in an automobile part and then hot-stamping the obtained steel sheets has been practically used (see patent documents 1 and 2). According to this method, the number of press dies can be reduced, only a specific portion of the overlapped hot press-formed body can be locally reinforced, and the part thickness is not unnecessarily increased, which contributes to weight reduction of the part. The blank produced by such overlapping and welding is referred to as an overlapped blank (also referred to as a tile blank). The blank is a metal plate such as a steel plate which is a material to be formed by press forming.

When the steel sheet to be used for the hot-press forming is an uncoated steel sheet, scale is generated on the surface of the produced hot-press member by high-temperature heating accompanied with hot-press forming. Therefore, there are problems as follows: after hot press forming, it is necessary to remove the scale formed by shot blasting, for example, or the corrosion resistance of the produced overlapped hot-pressed member is easily lowered. Further, as a specific problem when using non-plated steel sheets as a material for the superimposed blank, although shot blasting can be performed on the non-superimposed portion (hereinafter also referred to as "sheet-fed portion"), there is a problem that it is difficult to remove scale formed between the steel sheets in the superimposed portion (hereinafter also referred to as "superimposed portion") by shot blasting, and corrosion resistance is particularly liable to decrease.

If the steel sheet used for the overlapping is a plated steel sheet, the necessity of shot blasting the overlapped hot-pressed part after the hot press forming is eliminated. As the plated steel sheet used for hot pressing, a Zn-based plated steel sheet and an Al-based plated steel sheet are generally cited. In either of the Zn-based plating and the Al-based plating, after hot press heating, the Zn-based plating becomes Zn — Fe-based plating and the Al-based plating becomes Al — Fe-based plating by an alloying reaction in which Fe diffuses in the plating layer.

As shown in patent documents 2 and 3, a Zn-based plated steel sheet (i.e., a plated steel sheet containing 50 mass% or more of Zn (Zn plating, or Zn-based alloy plating such as Zn — Fe alloy, Zn — Ni alloy, Zn — Fe — Al alloy, etc.)) suppresses the generation of scale and eliminates the problem of requiring shot blasting. However, when a Zn-based plated steel sheet is used as a material of a lap-formed material and bending is performed on a lap portion in hot press forming, cracks may be generated in an iron base body, and a problem may occur in collision resistance. This is caused by a problem called liquid metal embrittlement in which Zn becomes liquid metal and intrudes into the iron matrix from the surface of the plating layer when zinc having a low melting point remains. Bending is a means for ensuring collision resistance from the aspect of shape, and bending at the overlapping portion is an extremely important method of applying a molded body to be overlapped.

As shown in patent documents 2 and 3, measures to cope with embrittlement of liquid metal in hot stamping using Zn-based plated steel sheets are generally as follows: a measure of performing a Zn-Fe alloying reaction to make the plating high melting point at the time of hot stamping heating, and a measure of lowering the forming temperature at the time of bend forming of hot stamping to wait for the zinc to solidify. However, as a specific problem when using a zinc-based plated steel sheet as a material for a clad material, there is a problem that the temperature rise rate and the cooling rate are both slow because the sheet thickness of the clad portion is thicker than that of the single-piece portion, and the Zn — Fe alloying reaction is difficult to proceed at the time of hot stamping. Further, there is a problem that the molding temperature at the time of hot press molding is as follows: if the overlapped portion is cooled, the single-piece portion is cooled quickly, and the martensite structure cannot be secured. Further, in the sheet portions, Zn forms a film of zinc oxide and evaporation of Zn is suppressed, but the corrosion resistance of the overlapped portions is lowered due to evaporation of Zn caused by oxygen deficiency in the atmosphere between the steel sheets of the overlapped portions, and the problem of embrittlement of the liquid metal is further increased.

In the Al-based plated steel sheet shown in patent document 4 (that is, a plated steel sheet containing 50 mass% or more of Al (Al plating, Al-based alloy plating such as Al — Si alloy, Al — Fe — Si alloy, or the like)), the formation of scale is suppressed in the same manner as Zn, and the problem of the necessity of shot blasting is solved.

Disclosure of Invention

Problems to be solved by the invention

However, when the Al-based plated steel sheet disclosed in patent document 4 is used as a material of a superimposed blank, there is a problem that the temperature increase rate of the superimposed portion is slow at the time of heating during hot stamping. As described in patent document 3, an Al — Fe alloying reaction of the plating layer by heating at the time of hot stamping is important for improving the corrosion resistance of the plating layer. When the temperature increase rate is low, the alloying reaction does not sufficiently proceed to the surface of the plating layer, and therefore, there is a problem that the corrosion resistance of the molded article after hot stamping is lowered. As a measure for solving this problem, it is conceivable to carry out the alloying reaction by extending the heating time in the hot stamping, but this measure has a problem that the productivity of the hot stamping is lowered, and the alloying reaction excessively proceeds in the single sheet portion to form a plating layer having a high Fe concentration, and the corrosion resistance of the plating layer is lowered in this case.

Therefore, as described above, in order to suppress the scale of the iron matrix and to prevent the liquid metal from becoming brittle, it is desired to solve the problem of the difference in the temperature increase rate between the overlapped portion and the blank portion and to improve the corrosion resistance of the plated layer after hot stamping in the aluminum-based plated steel sheet which is suitable as a material of the overlapped blank for hot stamping.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a superimposed blank for hot stamping, a method for producing a superimposed hot-stamped product, and a superimposed hot-stamped product, which can solve the problem of the difference in the temperature increase rate between the superimposed portion and the single-piece portion, and further improve the corrosion resistance of the plating layer after hot stamping, in the case where an aluminum-based plated steel sheet is used as a raw material.

Means for solving the problems

The present inventors have made extensive studies to solve the above problems, and as a result of focusing attention on the relationship between the plating deposit amount and the temperature increase rate of an aluminum-based plated steel sheet, they have found that the temperature increase rate increases as the plating deposit amount decreases. This is because the rate of temperature rise during hot press heating of the aluminum-based plated steel sheet is characterized in that the emissivity of a silvery white surface on which alloying reaction in which Fe is not diffused into the plating layer is not performed is low in terms of the appearance of the aluminum-based plated steel sheet, and the surface becomes black after the alloying reaction has progressed to the surface, and as a result, the emissivity of the surface becomes high. Therefore, it is considered that the amount of plating adhesion is small when the alloying rapidly proceeds to the surface, and the surface emissivity rapidly increases, so that the temperature increase rate increases.

Based on the above findings, the optimum plating deposit amount has been investigated for the problem of the difference in the temperature increase rate between the overlapped portion and the single-piece portion of the superimposed blank for hot stamping, and as a result, it has been found that it is important to reduce the plating deposit amount in the overlapped portion where the thickness of the steel sheet is large and the temperature increase rate is slow, and conversely, it is important to increase the plating deposit amount and reduce the temperature increase rate in the single-piece portion where the temperature increase rate is fast. More specifically, it has been found that in a superimposed blank for hot stamping, a problem of a difference in temperature increase rate between a single-piece portion and a superimposed portion can be solved by spot-welding a steel sheet (second steel sheet) having a small area corresponding to a superimposed portion to a steel sheet (first steel sheet) having a large area serving as a base of a molded body after hot stamping, for example, and forming the first steel sheet as an aluminum-based plated steel sheet having a large plating adhesion amount and the second steel sheet as an aluminum-based plated steel sheet having a small plating adhesion amount.

The gist of the present invention completed based on the above finding is as follows.

[1]A blank for hot stamping comprising: a first steel plate, and at least one second steel plate connected to the surface of the first steel plate via a weld point and having an area smaller than that of the first steel plate; the first steel sheet is a plated steel sheet having aluminum-based plating layers on both sides of the first steel sheet, the second steel sheet is a plated steel sheet having aluminum-based plating layers on both sides of the second steel sheet, and the amount of adhesion of the aluminum-based plating layers in the first steel sheet is W1 (g/m) in terms of the average amount of adhesion on both sides²) The amount of the aluminum plating deposited on the surface of the second steel sheet not in contact with the first steel sheet was W2 (g/m)²) Said W1 and said W2 are both at 20g/m²Above and 120g/m²The following ranges are satisfied, and the relationship between the following formulas (1) and (2) is satisfied.

More than or equal to 30 (W1-W2) less than or equal to 100 formula (1)

(W1/W2)²X (t1/t2) ≥ 1.5 formula (2)

In the formula (2), t1(mm) represents the plate thickness of the first steel plate, and t2(mm) represents the plate thickness of the second steel plate.

[2]According to [1]The overlapped blank for hot stamping is characterized in that the welding is spot welding, and the welding spot density of the spot welding is 1 spot/200 cm²The above.

[3] The superimposed blank for hot stamping according to [2], wherein the first steel sheet has a portion that becomes a flange portion made of only the first steel sheet after being subjected to hot stamping, at least a part of a superimposed portion of the first steel sheet and the second steel sheet has a portion that becomes a bent portion after being subjected to hot stamping, and at least one spot of the spot welding is present in the portion that becomes the bent portion after being subjected to hot stamping.

[4] The superimposed blank for hot stamping according to any one of [1] to [3], wherein a plate thickness t1(mm) of the first steel plate and a plate thickness t2(mm) of the second steel plate satisfy the relationship of the following formula (3).

(t2/t1) 2.0 or less formula (3)

[5] The superimposed blank for hot stamping according to any one of [1] to [4], wherein the aluminum-based plating layers of the first steel sheet and the second steel sheet have a two-layer structure of an aluminum layer and an aluminum-iron-based alloy layer in that order from the surface toward the base steel sheet, and a thickness d1(μm) of the aluminum-iron-based alloy layer of the first steel sheet and a thickness d2(μm) of the aluminum-iron-based alloy layer of the second steel sheet satisfy the following formula (4).

2 is less than or equal to (d2-d1) is less than or equal to 10 type (4)

[6] The superimposed blank for hot stamping according to any one of [1] to [5], wherein the second steel sheet further has a carbon-based coating film having an emissivity of 0.7 or more on a surface of the aluminum-based plating layer on a side not in contact with the first steel sheet.

[7]According to [1]～[6]The superimposed blank for hot stamping according to any one of the above claims, wherein the aluminum-based plating layer on the surface of the second steel sheet further comprises ZnO or TiO₂At least one of the above components and an amount of the adhesive to one surface of the substrate is 0.2g/m²The above film.

[8]A method for producing a stacked hot-stamped product, wherein, in the hot-stamping of heating a stacked blank for hot stamping comprising a first steel plate and at least one second steel plate having a smaller area than the first steel plate and connected to the surface of the first steel plate via a weld joint, and shaping the heated stacked blank after the heating, at least a part of the stacked portion of the first steel plate and the second steel plate is provided with a bendA bent portion obtained by working, wherein the first steel sheet is a plated steel sheet having aluminum-based plating layers on both sides of the first steel sheet, the second steel sheet is a plated steel sheet having aluminum-based plating layers on both sides of the second steel sheet, and the amount of adhesion of the aluminum-based plating layers in the first steel sheet is W1 (g/m) as the average amount of adhesion on both sides²) The amount of the aluminum plating deposited on the surface of the second steel sheet not in contact with the first steel sheet was W2 (g/m)²) Said W1 and said W2 are both at 20g/m²Above and 120g/m²The following ranges are satisfied, and the relationship between the following formulas (1) and (2) is satisfied.

More than or equal to 30 (W1-W2) less than or equal to 100 formula (1)

(W1/W2)²X (t1/t2) ≥ 1.5 formula (2)

In the formula (2), t1(mm) represents the plate thickness of the first steel plate, and t2(mm) represents the plate thickness of the second steel plate.

[9]according to [8]The method for producing the overlapped hot stamp-molded article, wherein the welding is spot welding, and the spot density of the spot welding is 1 spot/200 cm²The above.

[10] The method of producing a superimposed hot stamped and formed body according to item [9], wherein at least one spot welding of the spot welding is present in a portion which becomes the bent portion after the hot stamping.

[11] The method for producing a stacked hot stamped product according to any one of [8] to [10], wherein a plate thickness t1(mm) of the first steel plate and a plate thickness t2(mm) of the second steel plate satisfy a relationship of the following formula (3).

(t2/t1) 2.0 or less formula (3)

[12] The method for producing a stacked hot stamped steel according to any one of [8] to [11], wherein the aluminum-based plating layer of each of the first steel sheet and the second steel sheet has a two-layer structure of an aluminum layer and an aluminum-iron-based alloy layer in order from the surface toward the base steel sheet, and a thickness d1(μm) of the aluminum-iron-based alloy layer of the first steel sheet and a thickness d2(μm) of the aluminum-iron-based alloy layer of the second steel sheet satisfy the following formula (4).

2 is less than or equal to (d2-d1) is less than or equal to 10 type (4)

[13] The method for producing a stacked hot stamped steel according to any one of [8] to [12], wherein the second steel sheet further has a carbon-based coating film having an emissivity of 0.7 or more on a surface of the aluminum-based plating layer on a side not in contact with the first steel sheet.

[14]According to [8]～[13]The method for producing a stacked hot stamp-formed body according to any one of the above methods, wherein the aluminum plating layer on the surface of the second steel sheet further has ZnO or TiO on the surface thereof₂At least one of the above components and an amount of the adhesive to one surface of the substrate is 0.2g/m²The above film.

[15] A stacked hot stamp-formed body comprising: a first steel plate having a plate thickness of T1(mm), and at least one second steel plate connected to the surface of the first steel plate via a weld, having a smaller area than the first steel plate, and having a plate thickness of T2 (mm); the first steel sheet is a plated steel sheet having aluminum-based plating layers with an average plating thickness of K1(μm) on both sides of the first steel sheet, the second steel sheet is a plated steel sheet having aluminum-based plating layers with a plating thickness of K2(μm) on the surface on the side not in contact with the first steel sheet, and the overlap hot stamp-formed product satisfies the relationship between the following formula (11) and formula (12).

33 type (11) is more than or equal to 10 (K1-K2)

(K1/K2)²x (T1/T2) ≥ 1.5 formula (12)

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention described above, when an aluminum-based plated steel sheet is used as a material, it is possible to solve the problem associated with the difference in the temperature increase rate between the overlapped portion and the single-piece portion, and further improve the corrosion resistance of the plated layer after hot stamping.

Drawings

Fig. 1 is an explanatory view schematically showing an example of a superimposed blank for hot stamping, a method for producing a superimposed hot stamped product, and a superimposed hot stamped product according to an embodiment of the present invention.

Fig. 2 is an explanatory view schematically showing the structure of an aluminum-based plating layer having an aluminum layer and an aluminum-iron-based alloy layer of a superimposed blank for hot stamping according to the same embodiment.

FIG. 3 is a schematic view showing the structure of an aluminum-based plating layer having an aluminum layer and an aluminum-iron-based alloy layer, and further having a carbon-based coating film or containing ZnO and TiO on the surface thereof, of a green compact for hot stamping according to the same embodiment₂the structure of the coating film of (1).

Fig. 4 is an explanatory view schematically showing an example of a lap-formed product, a method for producing the lap-formed product, and a lap-formed product for hot stamping according to the same embodiment in which a portion to be a bent portion has a spot-welded spot after forming by increasing the spot-weld density of the spot-welding.

Fig. 5 is an explanatory view schematically showing the mounting positions of thermocouples as a method for measuring the temperature increase rate of the overlapped portion and the single-piece portion in the example.

Fig. 6 is an explanatory view schematically showing the shape of a molded article using a mold for evaluating the corrosion resistance of the molded article of the example.

Fig. 7 is an explanatory view schematically showing an example of a hot-stamping superimposed blank, a method of manufacturing a superimposed hot-stamped product, and a superimposed molded product according to an embodiment of the present invention in which the spot welding density of spot welding of the embodiment is increased as compared with fig. 5 and a spot welding of spot welding is provided in a portion which becomes a bent portion after molding.

Fig. 8 is an explanatory view schematically showing an example of a hot stamping superimposed blank, a method of manufacturing a superimposed hot stamped product, and a superimposed molded product according to an embodiment of the present invention in which the spot welding density of spot welding in the embodiment is reduced as compared with fig. 7 and a spot welding of spot welding is provided in a portion which becomes a bent portion after molding.

Detailed Description

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present specification and the drawings, the constituent elements having substantially the same functional configuration are denoted by the same reference numerals, and the repetitive description thereof is omitted.

Fig. 1 is an explanatory view schematically showing an example of a superimposed blank for hot stamping, a method for producing a superimposed hot stamped product, and a superimposed hot stamped product according to an embodiment of the present invention.

The superimposed blank for hot stamping according to the present embodiment is used as a material for a superimposed hot-stamped product.

As schematically shown in fig. 1, the hot-stamping blankmaking 4 according to the present embodiment is formed by welding (3) a first steel plate 1 and a second steel plate 2 having a smaller area than the first steel plate. In this case, in the superimposed blank 4 for hot stamping, a portion where the second steel plate 2 is superimposed is referred to as a superimposed portion 4a, and a portion where the second steel plate is not superimposed is referred to as a single-piece portion 4 b. In the hot-stamping blankmaking 4 according to the present embodiment, the second steel plate 2 is preferably disposed inside the first steel plate 1 so that there is no portion protruding from the first steel plate 1, as schematically shown in fig. 1.

Further, on the surface of the first steel sheet 1, both the surface 1a on the side contacting the second steel sheet 2 and the surface 1b on the side not contacting the second steel sheet 2 are subjected to aluminum plating (not shown), and similarly, the second steel sheet 2 is also subjected to aluminum plating (not shown) on both the surface 2a on the side contacting the first steel sheet 1 and the surface 2b on the side not contacting the first steel sheet 1.

As a method of manufacturing the overlapped hot stamped product according to the present embodiment, the overlapped blank 4 for hot stamping is heated to the Ac3 point or more in the heating furnace 5 to austenitize the steel sheet, and immediately after being taken out from the furnace, is press-formed and rapidly cooled by the die 6 to cause the martensitic transformation of the steel sheet. Thus, the superimposed blank 4 for hot stamping becomes the superimposed hot-stamped product 12 according to the present embodiment having excellent collision resistance. At this time, at least a part of the overlapped portion 4a includes a portion which becomes the bent portion 8 when the hot stamp-formed body 12 is overlapped.

In fig. 1, a molded product using a hat-shaped die is shown as an example of the superimposed hot stamped product 12, and the portions of the hot stamped product 12 are referred to as a crown portion 7, a bent portion 8 of the crown portion, a vertical wall portion 10, a flange portion 11, and a bent portion 9 of the flange portion.

Although the second steel plate 2 according to the present embodiment is disposed outside the crown portion 7 in fig. 1, the object of the present invention can be achieved even if the second steel plate 2 is disposed inside the crown portion 7.

(1. overlapping blank for hot stamping)

The hot-stamping preform 4 according to the present embodiment will be described in detail below.

As described above, the lap top 4 for hot stamping according to the present embodiment includes the first steel plate 1, and the second steel plate 2 connected to the surface of the first steel plate 1 via the welding points (i.e., welded to the first steel plate 1) and having an area smaller than that of the first steel plate 1; aluminum plating is applied to both surfaces of each of the first steel sheet 1 and the second steel sheet 2. That is, the first steel sheet 1 and the second steel sheet 2 according to the present embodiment are aluminum-plated steel sheets having aluminum-based plating layers on both surfaces of a steel sheet serving as a base material.

< parent material >

In the hot-stamping billet 4 according to the present embodiment, the chemical composition of the base material of each of the first steel sheet 1 and the second steel sheet 2 is not particularly limited. However, in order to obtain a tensile strength of, for example, 1500MPa or more (vickers hardness of about 400HV or more when the load is 9.81N), it is preferable to use a base material having a chemical composition of, in mass%, C: 0.19% or more and 0.5% or less, Si: 0.01% or more and 1.5% or less, Mn: 0.4% to 2%, Cr: 0.01% or more and 1.0% or less, Ti: 0.001% or more and 0.1% or more, B: 0.0005% or more and 0.005% or less, Nb: 0.1% or less, Mo, Ni, Cu, Co, W, Sn, V, Sb: respectively, less than 0.5%, Mg, Ca, Zr and REM: respectively less than 0.005%, and the balance of Fe and impurities. In the above chemical composition range, the chemical composition of the base material of the first steel plate 1 may be the same as or different from the chemical composition of the base material of the second steel plate 2.

The method for producing the aluminum-based plated steel sheet having the above chemical composition as the base material is not particularly limited, and for example, a steel sheet produced through a conventional iron making process, a steel making process, and a process of hot rolling, pickling, cold rolling, and sendzimir hot dip aluminum plating can be used.

In the present embodiment, the ratio (t2/t1) of the plate thickness t1(mm) of the first steel plate 1 to the plate thickness t2(mm) of the second steel plate 2 is selected as shown in the following formula (3), and is preferably 2.0 or less.

(t2/t1) 2.0 or less formula (3)

The reason why the above (3) is preferably satisfied will be described below.

In the characteristics required for the aluminum-based plated steel sheet in the present embodiment, it is important to further suppress the difference in the temperature increase rate between the overlapped portion (the temperature increase rate is slow) and the single-piece portion (the temperature increase rate is fast) which is a technical problem when used as the overlapped material, and therefore, it is considered important to suppress the thickness t2 of the second steel sheet 2 to a certain extent with respect to the thickness t1 of the first steel sheet 1. When the value of the ratio (t2/t1) exceeds 2.0, there is a high possibility that the plate thickness t2 of the second steel plate 2 becomes too large and the temperature increase rate of the overlapped portion becomes larger than that of the single-piece portion. The value of the ratio (t2/t1) is more preferably 0.9 or less. On the other hand, the lower limit of the ratio (t2/t1) is not particularly limited, but when the value of the ratio (t2/t1) is less than 0.3, there is no problem in suppressing the difference in the temperature increase rate, but it may be insufficient from the viewpoint of improving the collision resistance required for use as an automobile part by the overlapping portion. Therefore, the value of the ratio (t2/t1) is preferably 0.3 or more.

The plate thickness t1 of the first steel plate 1 and the plate thickness t2 of the second steel plate 2 can be measured using a micrometer. The thicknesses t1 and t2 are thicknesses including the thicknesses of the aluminum-based plating layers provided on both surfaces in addition to the thickness of the base material.

< aluminum plating layer >

The amount of aluminum plating applied to both surfaces of the first steel sheet 1 is W1 (g/m)²) And the amount W2 (g/m) of aluminum plating applied to both surfaces of the second steel sheet 2²) W1 and W2 are both 20g/m²Above and 120g/m²The following formulas (1) and (2) satisfy the relationship. Here, the adhesion amount W1 of the aluminum-based plating layer in the first steel sheet 1 represents the average adhesion amount on both surfaces of the first steel sheet 1. That is, the amount of aluminum plating deposited on one surface of the first steel sheet 1 was W1a and W1b (g/m), respectively²) When W1 is 0.5 × (W1a + W1 b). The amount of adhesion W2 of the aluminum plating layer on the second steel sheet 2 is not equivalent toThe amount of aluminum plating deposited on the surface of the first steel sheet 1 on the side of contact. In the second steel plate 2, when the produced superimposed blank is heated during hot stamping, the surface on the side not in contact with the first steel plate 1 becomes a surface exposed to a heat source for heating.

More than or equal to 30 (W1-W2) less than or equal to 100 formula (1)

(W1/W2)²X (t1/t2) ≥ 1.5 formula (2)

The characteristics required for the aluminum-based plating layer according to the present embodiment include: (a) the formation of Fe scale during hot stamping heating is suppressed, and (b) the occurrence of chipping and crushing of the plating layer due to the sliding (also referred to as powdering) of the plating layer during hot stamping molding is suppressed. The powdering is caused by a compressive stress applied to the plating layer on the surface inside the bent portion generated at the time of molding, a shear stress applied to the plating layer from the sliding of the mold at the time of molding, or the like. When the adhesion amount of the aluminum plating in each steel sheet W1, W2 is less than 20g/m²In this case, the thickness of the plating layer becomes thin, and the suppression of Fe scale is insufficient. Therefore, the adhesion amounts W1 and W2 of the aluminum plating layers in the steel sheets were 20g/m, respectively²The above. The amount of the aluminum plating layer deposited in each steel sheet W1 and W2 is preferably 30g/m independently²Above, more preferably 35g/m²The above. On the other hand, when the plating adhesion amounts W1 and W2 per one side of the steel sheets exceed 120g/m²In the case of this, there is a problem that the pulverization is not sufficiently suppressed. Therefore, in the present embodiment, the plating deposition amounts W1 and W2 are each independently 120g/m per one surface of each steel sheet²The following. The plating adhesion amounts W1 and W2 per surface of each steel sheet are preferably 115g/m²Hereinafter, more preferably 100g/m²The following.

The thickness (μm) of the aluminum plating layer in each steel sheet may be adjusted from the plating adhesion amount (g/m)²) The estimation depends on the chemical composition of the Al-based plating layer, but can be roughly determined from the following equation (5).

(thickness of plating layer) ═ (plating adhesion amount)/3 formula (5)

Further, as the characteristics required for the aluminum-based plating layer according to the present embodiment, there is (c) a property of suppressing the aluminum-based plating layer from being usedThe difference in the temperature increase rates between the overlapped portion (the temperature increase rate is slow) and the single portion (the temperature increase rate is fast) is a technical problem when the blanks are overlapped. As a measure for suppressing the difference in the temperature increase rate between the overlapped portion and the single-piece portion, the adhesion amount W2 of the aluminum-based plating layer in the second steel sheet 2 is set to a small adhesion amount with respect to the adhesion amount W1 of the aluminum-based plating layer in the first steel sheet 1, and specifically, as shown in the above formula (1), the difference (W1-W2) of the plating adhesion amounts is set to 30g/m²above and 100g/m²the following. By satisfying the relationship shown in the above formula (1), the alloying reaction of the plating layer which improves the emissivity at the time of heating in hot stamping can be rapidly advanced to the surface. When the difference (W1-W2) of the plating adhesion amounts is less than 30g/m²In this case, the above-described difference in temperature increase rate cannot be sufficiently improved. The difference (W1-W2) in the plating adhesion amount is preferably 35g/m²Above, more preferably 40g/m²The above. On the other hand, the upper limit of the difference in the plating deposit amounts (W1-W2) is not particularly limited from the viewpoint of suppressing the above-mentioned difference in the temperature rise rate, but the lower limit and the upper limit of the plating deposit amounts W1 and W2 per one surface of each steel sheet are 20g/m²、120g/m²Therefore, the upper limit in calculation is 100g/m². In addition, when the difference (W1-W2) of the plating adhesion amounts exceeds 100g/m²In the case, the corrosion resistance of the plating layer is lowered, which is not preferable. The difference (W1-W2) in the plating adhesion amount is preferably 90g/m²Hereinafter, more preferably 80g/m²The following. In consideration of satisfying the relationship expressed by the above equation (1), the upper limit value of the adhesion amount W2 of the aluminum-based plating layer in the second steel sheet 2 is substantially 90g/m²Is the upper limit.

The aluminum-based plating layer according to the present embodiment satisfies the relationship shown in the above (2) as well as the above formula (1). By satisfying the relationship shown in the above (2), the alloy reaction of the plating layer which improves the emissivity during heating in hot stamping can be rapidly advanced to the surface. In the above formula (2), the power of the ratio of the plate thicknesses (t1/t2) is 1, and the power of the ratio of the plating adhesion amounts (W1/W2) is 2. Thus, in the present invention, the positioning of the ratio of the plating adhesion amounts (W1/W2) is more important than the plate thickness ratio (t1/t 2).

When (W1/W2)²When the value of x (t1/t2) is less than 1.5, the difference in temperature increase rate cannot be sufficiently improved. (W1/W2)²The value of x (t1/t2) is preferably 2 or more, more preferably 2.5 or more. On the other hand, for (W1/W2)²The upper limit of (t1/t2) is not particularly limited. However, (W1/W2)²An excessive increase in the value of x (t1/t2), i.e., (W1/W2) or (t1/t2) results in an increase in material cost with an increase in W1 and t1, a decrease in corrosion resistance due to a decrease in W2, and a decrease in collision resistance due to a decrease in t 2. Thus, (W1/W2)²The value of x (t1/t2) is preferably 80 or less. (W1/W2)²The value of x (t1/t2) is more preferably 60 or less.

Fig. 2 schematically shows the layer structure on one surface side of the plated steel sheet 13 provided with the aluminum-based plating layer described in the present embodiment. A more preferable layer structure of the aluminum-based plating layer according to the present embodiment, which is selectively realized, will be described below with reference to fig. 2.

In a case where the plating structure is schematically shown on one surface side of the plated steel sheet 13 treated with the aluminum-based plating layer, the aluminum-based plating layer applied to the first steel sheet 1 and the second steel sheet 2 preferably has a two-layer structure of an aluminum layer 14 and an aluminum-iron-based alloy layer 15 in order from the surface toward the base material 16. Here, the thickness d1(μm) of the aluminum-iron alloy layer 15 of the first steel sheet 1 is preferably 1 μm or more, and more preferably 2 μm or more. The thickness d2(μm) of the aluminum-iron alloy layer 15 of the second steel sheet 2 is preferably 2 μm or more, and more preferably 3 μm or more. On the other hand, the thickness d1 of the aluminum-iron alloy layer 15 of the first steel sheet 1 is preferably 9 μm or less, and more preferably 8 μm or less. The thickness d2 of the aluminum-iron alloy layer 15 of the second steel sheet 2 is preferably 10 μm or less, and more preferably 9 μm or less.

The aluminum-based plating layers applied to the first steel sheet 1 and the second steel sheet 2 have a two-layer structure, and the difference (d2-d1) between the thickness d1(μm) of the aluminum-iron-based alloy layer 15 of the first steel sheet 1 and the thickness d2(μm) of the aluminum-iron-based alloy layer 15 of the second steel sheet 2 is as shown in the following formula (4), and more preferably 2 μm to 10 μm.

2 is less than or equal to (d2-d1) is less than or equal to 10 type (4)

This is because, as the characteristics required for the aluminum-based plating according to the present embodiment, it is possible to suppress the difference in the temperature increase rate between the overlapped portion (the temperature increase rate is slow) and the single portion (the temperature increase rate is fast). In this case, in order to allow the alloying reaction of the plating layer, which increases the emissivity during heating in hot stamping, to rapidly proceed to the surface in the overlapped portion, it is preferable to increase the thickness d2 of the aluminum-iron alloy layer 15 of the second steel sheet 2 from before heating in hot stamping, while the alloying reaction is rather retarded in the single portion, and therefore the thickness d1 of the aluminum-iron alloy layer 15 of the first steel sheet 1 is preferably reduced. When the difference in the thickness of the aluminum-iron alloy layer 15 (d2-d1) is less than 2 μm, the above-mentioned difference in the temperature increase rate cannot be sufficiently improved. The difference in the thickness of the aluminum-iron alloy layer 15 (d2-d1) is more preferably 3 μm or more. On the other hand, the upper limit of the difference (d2-d1) in the thickness of the aluminum-iron alloy layer 15 is not particularly limited from the viewpoint of the difference in the temperature increase rate, but when the value of the thickness d2 of the aluminum-iron alloy layer 15 exceeds 10 μm, excessive alloying proceeds, and the pulverization in the molded portion during hot stamping becomes severe, and the upper limit is 10 μm as described above. The difference in the thickness of the aluminum-iron alloy layer 15 (d2-d1) is more preferably 8 μm or less.

As a method of processing aluminum-based plating into a steel sheet, according to a conventional hot dip plating method, an aluminum-based plated steel sheet with an adjusted amount of deposit can be produced by immersing the steel sheet in a hot dip aluminum plating bath and performing gas wiping with nitrogen gas, the atmosphere, or the like. As a result, the al — Fe alloy layer 15 is inevitably formed at the interface between the plating layer and the base steel sheet (base material 16 in the present embodiment) by elution of Fe during hot dip plating. The thickness of the formed aluminum-iron-based alloy layer 15 can be increased by extending the immersion time in hot dip plating.

The chemical composition of the hot dip aluminum plating bath for forming the aluminum-based plating layer is not particularly limited. However, the content of Al in the hot-dip aluminizing bath used for formation is preferably 80 mass% or more from the viewpoint of excellent heat resistance, and the content of Si in the hot-dip aluminizing bath is preferably 2 mass% or more from the viewpoint of easy control of the thickness of the aluminum-iron alloy layer 15. If the Si content is less than 2 mass%, the aluminum-iron alloy layer 15 becomes too thick and formability is degraded. On the other hand, if the Si content of the hot dip aluminizing bath exceeds 15 mass%, the alloying at the time of hot stamping heating becomes slow, and the productivity of hot stamping decreases. Therefore, the Si content of the hot dip aluminum plating bath is preferably 15 mass% or less.

When Si is contained in an amount of 2 mass% to 15 mass%, the aluminum layer 14 has a eutectic structure of Al and Si based on a phase diagram. In the case of the hot dip coating method, Fe may be inevitably contained in an amount of 1 mass% or more as a component eluted from the steel sheet. Examples of the other inevitable impurities include elements such as Cr, Mn, V, Ti, Sn, Ni, Cu, W, Bi, Mg, and Ca, which are derived from components eluted from a hot dip coating facility and impurities in an ingot of a hot dip aluminizing bath, and these elements may be contained in an amount of less than 1 mass%.

The chemical composition of the aluminum-iron alloy layer 15 includes a θ phase (FeAl) which is a binary alloy of Al and Fe₃) Eta phase (Fe)₂Al₅) Zeta phase (FeAl)₂)、Fe₃Al, and Fe-based BCC phases (α 2, α), and the like, and the aluminum-iron-based alloy layer 15 is formed by a combination of these plating phases. The chemical composition of the aluminum-iron alloy layer 15 when Si is contained is τ 1-Al₂Fe₃Si₃、τ2-Al₃FeSi、τ3-Al₂FeSi、τ4-Al₃FeSi₂、τ5-Al₈Fe₂Si、τ6-Al₉Fe₂Si₂、τ7-Al₃Fe₂Si₃、τ8-Al₂Fe₃Si₄、τ10-Al₄Fe_1.7Si、τ11-Al₅Fe₂Si, etc., mainly consisting of τ 5.

In addition, as a method for determining the amount of the aluminum plating deposited, for example, a sodium hydroxide-hexamethylenetetramine/hydrochloric acid stripping gravimetric method is cited. Specifically, the following are measured in accordance with JIS G3314: 2011A predetermined area S (m) is prepared²) (for example, 50X 50mm) in advance, and the weight w1(g) was measured. Then, the plate was immersed in an aqueous sodium hydroxide solution and an aqueous hydrochloric acid solution containing hexamethylenetetramine in this order until the foam caused by the dissolution of the plating layer disappeared, immediately washed with water and the weight w2 was measured again(g) In that respect At this time, the amount Wp (g/m) of the aluminum plating layer deposited²) Can be obtained from the following equation (6).

Wp ═ w1-w2)/S formula (6)

When the test piece is small in size, the cross section of the plating layer is observed with an optical microscope (area: 100. mu. m.times.100 μm), the thickness of the plating layer is measured in three visual fields in the same manner, and the average value of the thicknesses measured in the three visual fields is obtained by converting the thickness into the deposit amount using the formula (5). The thickness of the plating layer measured at this time is the total thickness of the aluminum layer 14 and the thickness of the aluminum-iron alloy layer 15 shown in fig. 2.

The thickness of aluminum-iron alloy layer 15 is obtained by observing the cross section of the plated layer without etching with an optical microscope (area: 100. mu. m.times.100 μm) in the same manner, measuring the thickness of aluminum-iron alloy layer 15 in three visual fields in the same manner, and determining the average value of the thicknesses measured in the three visual fields.

< coating layer >

FIG. 3 schematically shows the aluminum-based plating layer according to the present embodiment, and the layer structure on one surface side of the plated steel sheet 18 having a carbon-based coating film further provided on the surface thereof, or the plated steel sheet having ZnO or TiO-containing layer₂The coated plated steel sheet 18' of (2) has a layer structure on one surface side. The aluminum-based plating layer according to the present embodiment selectively formed and the surface thereof further having a carbon-based coating film or containing ZnO or TiO₂A more preferable layer structure of the coating film of (3) will be described below with reference to fig. 3.

[ coating layer (carbon series) ]

In the second steel sheet 2, it is preferable that a carbon-based coating layer 17 having an emissivity of 0.7 or more is further provided on the surface of the aluminum-based plating layer on the surface not in contact with the first steel sheet 1. The aluminum-based plating layer in this case has the carbon-based coating layer 17, the aluminum layer 14, and the aluminum-iron-based alloy layer 15 on the base material 16 in this order from the surface toward the base material 16, as described above. In order to suppress a difference in the temperature increase rate between the overlapped portion (the temperature increase rate is slow) and the single-piece portion (the temperature increase rate is fast) which is a technical problem when used as a superimposed material, the emissivity of the carbon-based coating layer 17 is preferably set to 0.7 or more. If the emissivity is less than 0.7, the improvement effect is insufficientAnd (4) dividing. The upper limit of emissivity is set to 1 in principle. Examples of the component having a high emissivity include metal oxides and metal nitrides, but for example, a carbon-based coating containing carbon black is preferable. By providing a carbon-based coating containing carbon as a main component, the coating burns as CO during heating in hot stamping₂And the like, and thus the corrosion resistance of the molded article after hot stamping can be prevented from being lowered.

As a method for determining the emissivity, for example, when the radiation temperature of the sample is measured by infrared thermography (Nippon ionics co., ltd, G100EX) and the temperature of the sample is also measured by a K-type thermocouple, the emissivity can be determined by obtaining the emissivity at a temperature at which the radiation temperature most coincides with the temperature measured by the thermocouple. Further, as a method of determining the carbon-based coating layer 17, the following method can be mentioned: as a result of analyzing the depth direction of the coating with a high-frequency glow discharge emission surface analyzer (GDS, HORIBA, ltd.), the presence of the carbon-based coating layer 17 was determined when the carbon element (C) was detected at an element concentration of 20 mass% or more. The thickness of the carbon-based coating layer 17 is not particularly limited as long as the emissivity is 0.7 or more, and is preferably 0.2 μm or more, and more preferably 0.5 μm or more, from the viewpoint of ease of industrial processing of the coating treatment. On the other hand, the carbon-based coating layer 17 is preferably 5 μm or less, more preferably 3 μm or less, because the effect of improving emissivity is saturated and uneconomical, and the adhesion between the coating and the steel sheet is reduced. The thickness of the carbon-based coating layer 17 can be measured by the depth direction analysis using the GDS.

[ coating layer (ZnO, TiO)₂)]

preferably, the surface of the aluminum plating layer formed on the surface of the second steel sheet 2 further has ZnO or TiO₂Is composed of at least one of the above components and has an adhesion amount per one surface of 0.2g/m²The above coating layer 17'. The adhesion amount herein means an adhesion amount in terms of metal Zn or metal Ti per unit area. ZnO and TiO₂An oxide having an improved emissivity and a good infrared absorption. Therefore, by providing this coating layer 17', it is possible to suppress the use as a superimposed blankthe difference in the temperature increase rates between the overlapping portion (slow temperature increase rate) and the single portion (fast temperature increase rate) is a technical problem in the case of time. In particular, ZnO and TiO as oxides are compared with the case where the carbon-based coating layer 17 is burned in the hot press heating process₂Can remain even during heating. Therefore, the coating layer 17' can further contribute to the improvement of emissivity at high temperatures. When the adhering amount of the coating layer 17' is less than 0.2g/m²In this case, the effect of suppressing the temperature increase rate difference may not be sufficiently expected. The amount of the coating layer 17' to be adhered is more preferably 0.3g/m²The above. On the other hand, the upper limit of the amount of adhesion of the coating layer 17' is not particularly limited, and an excessive amount of adhesion saturates the effect and increases the coating cost, so that it is not practical, and in addition, ZnO and TiO may remain after the hot press heating₂Therefore, corrosion resistance and the like are reduced. Therefore, the amount of the coating layer 17' to be adhered is more preferably 3g/m²The following. As a means for determining ZnO and TiO₂The amount of adhesion of (a) can be determined by elemental analysis from the surface using an X-ray fluorescence analyzer (ZSX Primus, manufactured by RIGAKU Co., Ltd.) to quantify the amount of Zn and Ti.

The carbon-based coating layer 17 described above has ZnO or TiO₂The method for treating the coating layer 17' of (a) is not particularly limited, and for example, water-dispersible carbon black (for example, RCF #52 manufactured by mitsubishi chemical corporation), ZnO (for example, c.i. kasei co., ltd. manufactured, Nano Tek), or TiO may be prepared₂An aqueous coating liquid (for example, c.i. kasei co., ltd. and Nano Tek) dispersed in water, and the hot dip aluminum plating treatment is performed, followed by coating with a roll coater and drying and baking.

The aluminum-based plating layer according to the present embodiment may include the carbon-based coating layer 17 and ZnO or TiO₂The coating layer 17'. In this case, the carbon-based coating layer 17 may have ZnO or TiO₂The order of disposing the coating layer 17' is not particularly limited, and the carbon-based coating layer 17 may be formed to have ZnO or TiO₂the upper layer of the coating layer 17' of (2) may have ZnO or TiO₂The coating layer 17' of (a) is located on the upper layer of the carbon-based coating layer 17.

The carbon-based coating film 17 contains ZnO and TiO₂The coating layers 17' may be provided on both surfaces of the steel sheet serving as the base material, but are more preferably provided only on the surface of the steel sheet serving as the base material that is exposed to the heat source during heating in hot stamping.

< welding >

In a lap-joint material for hot stamping in which a first steel sheet 1 and a second steel sheet 2 are overlapped and welded, the welding is preferably spot welding with a spot weld density of 1 spot/200 cm²The above. The reason for this will be described below.

By making the first steel plate 1 and the second steel plate 2 in good contact with each other in the overlapped portion, it is possible to improve heat transfer and suppress a difference in temperature increase rate between the overlapped portion (a slow temperature increase rate) and the single-piece portion (a fast temperature increase rate) which is a technical problem when used as a superimposed billet.

As the type of welding, spot welding, seam welding, brazing welding, laser welding, plasma welding, arc welding, or the like can be selected, but in order to make the overlapped portions well contact with each other, it is preferable to make spot welding in which direct joining can be performed by bringing the steel sheets into contact with each other at a plurality of points until the inside of the overlapped portions and applying pressure between the steel sheets.

As described above, the spot welding density of the spot welding is preferably 1 spot/200 cm²The above. If the density of welding spots is less than 1 point/200 cm²The contact between the steel sheets is insufficient, and the temperature rise of the overlapped portion is not sufficiently improved. The spot density of spot welding is more preferably 1 spot/40 cm²The above. On the other hand, although the upper limit of the spot density in spot welding is not particularly limited, if the density is too high, the welding current is shunted and welding becomes difficult, and therefore, it is preferable that the density is 1 spot/1 cm²The following.

Spot weld density (spot/cm) of the above spot welding²) The number of spot welds in the second steel plate 2 processed as a blank is divided by the area of the second steel plate 2.

Preferably, at least one spot welding point of the spot welding is present in a portion which becomes a bent portion after the hot press forming. In order to suppress the difference in the temperature increase rate between the overlapped portion (the temperature increase rate is slow) and the single-piece portion (the temperature increase rate is fast) which is a technical problem when used as a superimposed billet, it is important to bring the first steel plate 1 and the second steel plate 2 in the overlapped portion into good contact. Here, as shown in fig. 4, the hot stamping is performed using the superimposed blank 22 for hot stamping to manufacture a hot stamped and superimposed molded body 26. In this case, during hot press forming, stress is more likely to be applied to the bent portion than to the crown portion and the vertical wall portion, and therefore, voids are likely to be generated, and a sufficient cooling rate cannot be obtained during cooling of the die, so that hardness is reduced, and collision resistance is reduced. Therefore, by providing a spot welding point (welding portion 24) for spot welding at the bent portion of the hot stamp-formed body 26, the void of the bent portion can be suppressed. Therefore, in the superimposed blank for hot stamping 22, as shown in fig. 4, a spot (welded portion 20) is preferably applied to a portion which becomes a bent portion after hot stamping. In the present embodiment, after hot press forming, the spot welding (the welded portion 19) of spot welding may be performed on the portion to be the vertex portion, and the spot welding (the welded portion 21) of the spot welded portion may be performed on the portion to be the vertical wall portion. As a result, after spot welding, a spot of spot welding (welded portion 23) is disposed at the vertex of the hot stamped-formed part 26, and a spot of spot welding (welded portion 25) is disposed at the vertical wall of the hot stamped-formed part 26.

(2. overlapped hot stamp-molded article and method for producing the same)

In the method of manufacturing a hot stamped steel according to the present embodiment, as shown in fig. 1, the superimposed blank 4 for hot stamping is heated, and when the blank is formed immediately after the heating, the superimposed hot stamped steel 12 according to the present embodiment is manufactured by providing a bent portion obtained by bending at least a part of the superimposed portion.

The heating temperature is not particularly limited, but is usually set to a temperature range of Ac3 point (for example, 800 ℃) or more and 1000 ℃ or less, and when molding is performed immediately after heating, the hot-stamped steel 12 having excellent collision resistance can be obtained by cooling with a cooling medium such as a die or water. The heating temperature is the maximum temperature of the steel sheets in the overlapped portion, and examples of the heating method include heating by an electric furnace, a gas furnace, a far-infrared furnace, a near-infrared furnace, or the like, electric heating, high-frequency heating, induction heating, and the like.

The overlapped hot stamped steel 12 of the present embodiment manufactured as described above includes the first steel plate having a plate thickness of T1(mm), and at least one second steel plate that is overlapped and welded to the first steel plate, has a smaller area than the first steel plate, and has a plate thickness of T2 (mm). Here, the first steel sheet in the stacked hot stamped steel 12 is a plated steel sheet having aluminum plating layers with an average plating thickness K1(μm) on both sides of the first steel sheet. The second steel sheet of the superimposed hot stamped steel 12 is a plated steel sheet having an aluminum-based plating layer with a plating thickness K2(μm) on the surface on the side not in contact with the first steel sheet. The plating thickness of the aluminum-based plating layer on the surface of the second steel sheet on the side in contact with the first steel sheet is not particularly limited.

Here, the average plating thickness K1 of the aluminum-based plating layer in the first steel sheet is preferably 20 μm or more, and more preferably 25 μm or more. The plating thickness K2 of the aluminum-based plating layer in the second steel sheet on the side not in contact with the first steel sheet is preferably 10 μm or more, and more preferably 15 μm or more. On the other hand, the average plating thickness K1 of the aluminum-based plating layer in the first steel sheet is preferably 55 μm or less, and more preferably 50 μm or less. The plating thickness K2 of the aluminum-based plating layer in the second steel sheet on the side not in contact with the first steel sheet is preferably 45 μm or less, and more preferably 40 μm or less. When the average plating thicknesses K1 and K2 are within the above ranges, the corrosion resistance of the stacked hot stamped steel body 12 can be kept in a good state.

Further, the stacked hot stamp-formed body 12 according to the present embodiment satisfies the relationship between the following formula (11) and formula (12).

33 type (11) is more than or equal to 10 (K1-K2)

(K1/K2)²X (T1/T2) ≥ 1.5 formula (12)

in the overlapped hot stamped body 12 described in the present embodiment, when the difference in plating thickness (K1-K2) is less than 10, it is difficult to maintain the corrosion resistance of the overlapped hot stamped body 12 in a good state. The difference in plating thickness (K1-K2) is preferably 12 μm or more, and more preferably 14 μm or more. On the other hand, if the difference in plating thickness (K1-K2) exceeds 33, the corrosion resistance of the plating layer decreases, which is not preferable. The difference in plating thickness (K1-K2) is preferably 30 μm or less, more preferably 27 μm or less.

The stacked hot stamp-formed body 12 according to the present embodiment satisfies the relationship shown in the above (12) as well as the above formula (11). By satisfying the relationship shown in the above (12), the alloying reaction of the plating layer that improves the emissivity during heating in hot stamping proceeds rapidly to the surface, resulting in the superimposed hot stamped steel 12 exhibiting good corrosion resistance.

When (K1/K2)²When the value of x (T1/T2) is less than 1.5, the difference in temperature rise rate cannot be sufficiently improved, and it is difficult to maintain good corrosion resistance. (K1/K2)²The value of X (T1/T2) is preferably 2 or more, more preferably 2.5 or more. On the other hand, for (K1/K2)²The upper limit of X (T1/T2) is not particularly limited. However, (K1/K2)²an excessive increase in the value of x (T1/T2), i.e., an increase in (K1/K2) or (T1/T2), results in an increase in material cost with an increase in K1 and T1, a decrease in corrosion resistance due to a decrease in K2, and a decrease in collision resistance due to a decrease in T2. Thus, (K1/K2)²The value of X (T1/T2) is preferably 80 or less. (K1/K2)²The value of X (T1/T2) is more preferably 60 or less.

Here, the plating thicknesses K1 and K2 can be determined by observing the cross section of the plated layer after nital etching with an optical microscope (area: 100 μm × 100 μm), measuring the plating thicknesses in three visual fields, and averaging the plating thicknesses measured in the three visual fields. The plating thickness of the first steel sheet is measured at the single-piece portion, but the plating thickness of the first steel sheet is measured at the single-piece portion, from the viewpoint of a high temperature increase rate, a longest heating time in hot stamping, and a tendency to deteriorate corrosion resistance.

When the stacked hot stamp-molded body 12 of the present embodiment is used as an automobile part, welding, a phosphoric acid-based chemical conversion treatment, electrodeposition coating, or the like is generally performed and used. Therefore, for example, a zinc phosphate coating and a phosphoric acid coating may be formed on the surface of the hot stamped steel body 12 by a phosphoric acid chemical conversion treatment, or an organic coating of 5 μm or more and 50 μm or less may be formed on the surface by electrodeposition coating. After the electrodeposition coating, coating such as an intermediate coating or a surface coating may be further performed to improve appearance quality and corrosion resistance.