阅读说明：本技术 热冲压成形品和热冲压用钢板、以及它们的制造方法 (Hot press-formed product, steel sheet for hot press, and methods for producing these ) 是由 芳贺纯 于 2020-03-31 设计创作，主要内容包括：该热冲压成形品具有规定的化学组分。所述热冲压成形品的金属组织以面积％计包含铁素体：超过60.0％、马氏体：0％以上且小于20.0％、以及贝氏体：0％以上且小于20.0％。所述热冲压成形品的拉伸强度小于700MPa,在170℃下实施20分钟的热处理时的所述拉伸强度的降低量即ΔTS为100MPa以下。(The hot press-formed product has a predetermined chemical composition. The metal structure of the hot press-formed product contains, in area%: more than 60.0%, martensite: 0% or more and less than 20.0%, and bainite: more than 0% and less than 20.0%. The hot press-formed product has a tensile strength of less than 700MPa, and a decrease Δ TS in the tensile strength when heat-treated at 170 ℃ for 20 minutes is 100MPa or less.)

1. A hot press-formed article, wherein,

all or a part of the hot press-formed article has the following chemical composition in mass%:

c: more than 0.001% and less than 0.090%,

Si: less than 2.50 percent of,

Mn: more than 0.01 percent and less than 0.50 percent,

P: less than 0.200 percent,

S: less than 0.0200%,

sol.Al：0.001～2.500％、

N: less than 0.0200%,

Cr: more than 0.01 percent and less than 2.00 percent,

Ti：0～0.300％、

Nb：0～0.300％、

V：0～0.300％、

Zr：0～0.300％、

Mo：0～2.00％、

Cu：0～2.00％、

Ni：0～2.00％、

B：0～0.0200％、

Ca：0～0.0100％、

Mg：0～0.0100％、

REM：0～0.1000％、

Bi: 0 to 0.0500%, and

the rest is as follows: fe and impurities in the iron-based alloy, and the impurities,

the metal structure comprises in area%:

ferrite: over 60.0%,

Martensite: more than 0% and less than 20.0%,

Bainite: more than 0 percent and less than 20.0 percent,

the tensile strength is less than 700MPa,

the amount of decrease in tensile strength, i.e., Δ TS, when heat-treated at 170 ℃ for 20 minutes is 100MPa or less.

2. The hot press-formed article according to claim 1,

the chemical component contains 1 or 2 or more selected from the group consisting of the following components in mass%:

Ti：0.001～0.300％、

Nb：0.001～0.300％、

V：0.001～0.300％、

Zr：0.001～0.300％、

Mo：0.001～2.00％、

Cu：0.001～2.00％、

Ni：0.001～2.00％、

B：0.0001～0.0200％、

Ca：0.0001～0.0100％、

Mg：0.0001～0.0100％、

REM: 0.0001 to 0.1000%, and

Bi：0.0001～0.0500％。

3. the hot press-formed article according to claim 1 or 2,

the chemical components contain, in mass%:

mn: more than 0.01% and less than 0.30%.

4. The hot press-formed article according to any one of claims 1 to 3,

the surface is provided with a plating layer.

5. A steel sheet for hot stamping, wherein the chemical composition is, in mass%:

c: more than 0.001% and less than 0.090%,

Si: less than 2.50 percent of,

Mn: more than 0.01 percent and less than 0.50 percent,

P: less than 0.200 percent,

S: less than 0.0200%,

sol.Al：0.001～2.500％、

N: less than 0.0200%,

Cr: more than 0.01 percent and less than 2.00 percent,

Ti：0～0.300％、

Nb：0～0.300％、

V：0～0.300％、

Zr：0～0.300％、

Mo：0～2.00％、

Cu：0～2.00％、

Ni：0～2.00％、

B：0～0.0200％、

Ca：0～0.0100％、

Mg：0～0.0100％、

REM：0～0.1000％、

Bi: 0 to 0.0500%, and

the rest is as follows: fe and impurities in the iron-based alloy, and the impurities,

the metal structure contains iron carbide, and the content of Mn and the content of Cr in the iron carbide satisfy the following formula (i):

[Mn]_θ+[Cr]_θ>0.8…(i)

wherein, the meaning of each symbol in the above formula is as follows:

[Mn]_θ: a Mn content in the iron carbide in atomic% where the total content of Fe, Mn, and Cr contained in the iron carbide is 100 atomic%,

[Cr]_θ: a Cr content in the iron carbide in atomic% where the total content of Fe, Mn, and Cr contained in the iron carbide is 100 atomic%.

6. The steel sheet for hot stamping according to claim 5, wherein,

the chemical component contains 1 or 2 or more selected from the group consisting of the following components in mass%:

Ti：0.001～0.300％、

Nb：0.001～0.300％、

V：0.001～0.300％、

Zr：0.001～0.300％、

Mo：0.001～2.00％、

Cu：0.001～2.00％、

Ni：0.001～2.00％、

B：0.0001～0.0200％、

Ca：0.0001～0.0100％、

Mg：0.0001～0.0100％、

REM: 0.0001 to 0.1000%, and

Bi：0.0001～0.0500％。

7. the steel sheet for hot stamping according to claim 5 or 6, wherein,

the chemical components contain, in mass%:

mn: more than 0.01% and less than 0.30%.

8. The steel sheet for hot stamping according to any one of claims 5 to 7, wherein,

the surface is provided with a plating layer.

9. A method for producing a hot press-formed article according to any one of claims 1 to 3, comprising:

heating the steel sheet for hot stamping as claimed in any of claims 5 to 7 to a temperature exceeding Ac₃A heating step of heating the dots at a temperature T DEG C; and

and a hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C with respect to the steel sheet for hot stamping after the heating step.

10. A method for producing a hot press-formed article according to any one of claims 1 to 3, comprising:

a joining step of joining the steel sheet for hot stamping according to any one of claims 5 to 7 and a steel sheet for joining to produce a joined steel sheet;

connect the saidHeating the joined steel sheet after the joining step to a temperature exceeding Ac of the steel sheet for hot stamping₃A heating step of heating the dots at a temperature T DEG C; and

and a hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C for the joined steel sheet after the heating step.

11. A method for producing a hot press-formed article according to claim 4, comprising:

heating the steel sheet for hot stamping according to claim 8 to a temperature exceeding Ac₃A heating step of heating the dots at a temperature T DEG C; and

and a hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C with respect to the steel sheet for hot stamping after the heating step.

12. A method for producing a hot press-formed article according to claim 4, comprising:

a joining step of joining the steel sheet for hot stamping according to claim 8 and a steel sheet for joining to produce a joined steel sheet;

heating the joined steel sheet after the joining step to a temperature exceeding Ac of the steel sheet for hot stamping₃A heating step of heating the dots at a temperature T DEG C; and

and a hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C for the joined steel sheet after the heating step.

13. A method for manufacturing a steel sheet for hot stamping according to any one of claims 5 to 8, comprising:

a hot rolling step of hot rolling a slab, and then coiling the slab at a temperature of 800 ℃ or lower to produce a hot-rolled steel sheet, wherein the slab has a chemical composition in mass% of C: 0.001% or more and less than 0.090%, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% of the following, sol.al: 0.001-2.500%, N: 0.0200% or less, Cr: 0.01% or more and less than 2.00%, Ti: 0-0.300%, Nb: 0-0.300%, V: 0-0.300%, Zr: 0-0.300%, Mo: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, B: 0-0.0200%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0 to 0.1000%, Bi: 0-0.0500%, and the remainder: fe and impurities;

a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet to a temperature range exceeding 650 ℃ to produce a hot-rolled annealed steel sheet; and

and a cold rolling step of cold rolling the hot-rolled annealed steel sheet to produce a cold-rolled steel sheet.

14. The method of manufacturing a steel sheet for hot stamping according to claim 13, comprising:

and a plating step of optionally continuously annealing the cold-rolled steel sheet after the cold-rolling step and then plating the annealed steel sheet.

Technical Field

The present invention relates to a hot-stamped product, a steel sheet for hot stamping, and methods for producing the same.

The present application claims priority based on Japanese application No. 2019-070211, filed on 1/4/2019, the contents of which are incorporated herein by reference.

Background

Today, the industrial technology field is highly differentiated, and materials used in each technology field are required to have special and high performance. For example, in the case of steel sheets for automobiles, high strength is required for improvement of fuel efficiency due to weight reduction of a vehicle body in consideration of global environment. When a high-strength steel sheet is applied to a vehicle body of an automobile, the thickness of the steel sheet can be reduced to reduce the weight of the vehicle body, and a desired strength can be imparted to the vehicle body.

However, in press forming, which is a step of forming a body member of an automobile, cracks and wrinkles are more likely to occur as the thickness of a steel sheet used is reduced. Therefore, excellent press formability is required for the steel sheet for automobiles.

Ensuring press formability is an opposite factor to increasing the strength of a steel sheet, and therefore it is difficult to satisfy these properties at the same time. Further, when a high-strength steel sheet is press-formed, the shape of the member is greatly changed by springback when the member is taken out from the die, and it is difficult to ensure the dimensional accuracy of the member. As described above, it is not easy to manufacture a high-strength vehicle body member by press forming.

As a method for manufacturing an ultrahigh-strength vehicle body member, for example, as disclosed in patent document 1, a technique of press-forming a heated steel sheet using a low-temperature press die has been proposed. This technique is called hot stamping, hot pressing, or the like, and is capable of manufacturing a member having a complicated shape with high dimensional accuracy because a steel sheet heated to a high temperature and made soft is press-formed. Further, since the steel sheet is rapidly cooled by contact with the die, the strength can be significantly improved at the same time as the press forming by quenching. For example, patent document 1 describes that a steel sheet having a tensile strength of 500 to 600MPa is hot-pressed to obtain a member having a tensile strength of 1400MPa or more.

In a vehicle body member, in a skeleton structural member such as a center pillar and a side member, in order to control a deformation state of the member at the time of a vehicle collision, a hard portion and a soft portion are often provided in the member.

As a method for manufacturing a member having a soft portion by hot stamping, patent document 2 discloses a method for softening a portion heated to a low temperature by locally changing the heating temperature of a steel sheet by induction heating or infrared heating.

Patent document 3 discloses a method of softening a steel sheet by attaching a heat insulating material to a part of the steel sheet and locally lowering the heating temperature when the steel sheet is furnace-heated.

Patent documents 4 and 5 disclose a method of changing the contact area between the steel sheet and the die during forming to locally change the cooling rate of the steel sheet, thereby softening a portion having a low cooling rate.

Patent document 6 discloses a technique of performing hot stamping using a so-called tailor welded material in which two blank plates are welded and joined.

In hot stamping, generally, a steel sheet is heated to an austenite region and then cooled at a cooling rate equal to or higher than a critical cooling rate, thereby forming a martensite single structure and increasing the strength of the martensite single structure. On the other hand, in the methods described in patent documents 2 to 5, as described above, the heating temperature or the cooling temperature of the steel sheet is locally lowered, and a structure other than martensite is locally generated to soften the steel sheet. However, since the fraction of the microstructure other than martensite sensitively reacts to and changes in the heating temperature and the cooling rate, patent documents 2 to 5 have a problem that the strength of the soft portion is unstable.

In the technique described in patent document 6, a steel sheet having low hardenability is used as one of the blanks, and thus the soft portion can be formed under a constant heating and cooling condition. However, although the microstructure and strength characteristics of the soft portion greatly depend on the composition of the steel sheet, patent document 6 does not consider the composition of the steel sheet having low hardenability.

In order to solve such a problem, patent documents 7 and 8 disclose a method of stabilizing the strength of a soft portion in a hot-stamped member composed of a hard portion and a soft portion or a hot-stamped member which is soft as a whole.

Specifically, patent document 7 discloses a 600 to 1200MPa class high-strength automotive member in which the C content is limited to a low level, a certain amount or more of quenching elements are contained, and the formation of ferrite, pearlite, and martensite is suppressed during cooling, and a method for manufacturing the same.

Further, patent document 8 discloses a hot stamped member having a tensile strength of 500MPa or more, in which the C content is limited to be low, Ti is contained, and the amount of martensite produced is controlled, and a method for manufacturing the same.

According to the techniques described in patent documents 7 and 8, the strength and elongation in the member can be improved. However, according to the studies of the present inventors, it is found that in the techniques described in patent documents 7 and 8, since a hard structure such as bainite or martensite is included in a metal structure, thermal stability is low, and strength may be reduced when a member is subjected to a paint baking treatment. Since the coating baking treatment is often performed on automobile parts, the techniques described in patent documents 7 and 8 leave room for improvement.

Prior art documents

Patent document

Patent document 1: japanese laid-open patent application No. 2002-102980

Patent document 2: japanese laid-open patent publication No. 2005-193287

Patent document 3: japanese patent application laid-open No. 2009-61473

Patent document 4: japanese unexamined patent publication No. 2003-328031

Patent document 5: international publication No. 2006/38868

Patent document 6: japanese laid-open patent publication No. 2004-58082

Patent document 7: japanese laid-open patent publication No. 2005-248320

Patent document 8: international publication No. 2008/132303

Disclosure of Invention

Technical problem to be solved by the invention

As described above, it is not easy to manufacture a soft member or a member including a soft portion by hot stamping. In particular, it has been difficult to produce a low-strength hot stamped member (molded article) having excellent thermal stability, which includes a soft portion in part or all thereof, by hot stamping.

An object of the present invention is to provide a hot press-formed product having excellent thermal stability, more specifically, a portion in which fluctuation of strength (tensile strength) before and after a paint baking treatment accompanying the paint baking treatment is small and the tensile strength is less than 700MPa, a steel sheet for hot press suitable as a material thereof, and a method for producing the same, while solving the above-mentioned problems.

Means for solving the problems

The present invention has been made to solve the above-described problems, and the main points of the present invention are the hot-stamped product and the steel sheet for hot stamping described below, and the methods for producing them.

(1) A hot press-formed article, wherein all or a part of the hot press-formed article has the following chemical composition in mass%: c: 0.001% or more and less than 0.090%, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% of the following, sol.al: 0.001-2.500%, N: 0.0200% or less, Cr: 0.01% or more and less than 2.00%, Ti: 0-0.300%, Nb: 0-0.300%, V: 0-0.300%, Zr: 0-0.300%, Mo: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, B: 0-0.0200%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0 to 0.1000%, Bi: 0-0.0500%, and the remainder: fe and impurities, the microstructure comprising, in area%: more than 60.0%, martensite: 0% or more and less than 20.0%, bainite: 0% or more and less than 20.0%, and a tensile strength of less than 700MPa, wherein the amount of decrease in the tensile strength, i.e., Δ TS, when heat-treated at 170 ℃ for 20 minutes is 100MPa or less.

(2) The hot press-formed article according to the above (1), wherein the chemical component contains 1 or 2 or more species selected from the group consisting of the following components in mass%: ti: 0.001 to 0.300%, Nb: 0.001-0.300%, V: 0.001 to 0.300%, Zr: 0.001 to 0.300%, Mo: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Ni: 0.001-2.00%, B: 0.0001-0.0200%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, REM: 0.0001 to 0.1000%, and Bi: 0.0001 to 0.0500%.

(3) The hot press-formed article according to the above (1) or (2), wherein the chemical component contains, in mass%, Mn: more than 0.01% and less than 0.30%.

(4) The hot press-formed article according to any one of the above (1) to (3), wherein the surface has a plated layer.

(5) A steel sheet for hot stamping, wherein the chemical composition is, in mass%: c: 0.001% or more and less than 0.090%, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% of the following, sol.al: 0.001-2.500%, N: 0.0200% or less, Cr: 0.01% or more and less than 2.00%, Ti: 0-0.300%, Nb: 0-0.300%, V: 0-0.300%, Zr: 0-0.300%, Mo: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, B: 0-0.0200%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0 to 0.1000%, Bi: 0-0.0500%, and the remainder: fe and impurities, and the metal structure contains iron carbide, and the Mn content and the Cr content in the iron carbide satisfy the following formula (i).

[Mn]_θ+[Cr]_θ>0.8…(i)

Wherein each symbol in the above formula has the following meaning.

[Mn]_θ: a Mn content in the iron carbide in atomic% where the total content of Fe, Mn, and Cr contained in the iron carbide is 100 atomic%,

[Cr]_θ: a Cr content in the iron carbide in atomic% where the total content of Fe, Mn, and Cr contained in the iron carbide is 100 atomic%.

(6) The steel sheet for hot stamping as set forth in the above (5), wherein the chemical component contains 1 or 2 or more species selected from the group consisting of the following components in mass%: ti: 0.001 to 0.300%, Nb: 0.001-0.300%, V: 0.001 to 0.300%, Zr: 0.001 to 0.300%, Mo: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Ni: 0.001-2.00%, B: 0.0001-0.0200%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, REM: 0.0001 to 0.1000%, and Bi: 0.0001 to 0.0500%.

(7) The steel sheet for hot stamping according to (5) or (6), wherein the chemical component contains, in mass%, Mn: more than 0.01% and less than 0.30%.

(8) The steel sheet for hot stamping as recited in any one of (5) to (7) above, wherein the surface has a plated layer.

(9) A method for producing a hot press-formed article according to any one of the above (1) to (3), comprising: heating the steel sheet for hot stamping as described in any of (5) to (7) above to a temperature exceeding Ac₃A heating step of heating the dots at a temperature T DEG C; and a hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C for the steel sheet for hot stamping after the heating step.

(10) A method for producing a hot press-formed article according to any one of the above (1) to (3), comprising: a joining step of joining the steel sheet for hot stamping and the steel sheet for joining as described in any one of (5) to (7) above to produce a joined steel sheet; heating the joined steel sheet after the joining step to a temperature exceeding Ac of the steel for hot stamping₃A heating step of heating the dots at a temperature T DEG C; and a hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C to the joined steel sheet after the heating step.

(11) A method for producing a hot press-formed article according to the above (4), comprising: heating the steel sheet for hot stamping as described in (8) to a temperature exceeding Ac₃A heating step of heating the dots at a temperature T DEG C; and the hot stamping after the heating stepA hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C.

(12) A method for producing a hot press-formed article according to the above (4), comprising: a joining step of joining the steel sheet for hot stamping according to (8) and the steel sheet for joining to produce a joined steel sheet; heating the joined steel sheet after the joining step to a temperature exceeding Ac of the steel sheet for hot stamping₃A heating step of heating the dots at a temperature T DEG C; and a hot stamping step of starting hot stamping at a temperature of not less than (T-80) DEG C to the joined steel sheet after the heating step.

(13) A method for manufacturing a steel sheet for hot stamping according to any one of the above (5) to (8), comprising: a hot rolling step of hot rolling a slab, and then coiling the slab at a temperature of 800 ℃ or lower to form a hot-rolled steel sheet, wherein the slab comprises the following chemical components in mass%: c: 0.001% or more and less than 0.090%, Si: 2.50% or less, Mn: 0.01% or more and less than 0.50%, P: 0.200% or less, S: 0.0200% of the following, sol.al: 0.001-2.500%, N: 0.0200% or less, Cr: 0.01% or more and less than 2.00%, Ti: 0-0.300%, Nb: 0-0.300%, V: 0-0.300%, Zr: 0-0.300%, Mo: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, B: 0-0.0200%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0 to 0.1000%, Bi: 0-0.0500%, and the remainder: fe and impurities; a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet to a temperature in a range exceeding 650 ℃ to produce a hot-rolled annealed steel sheet; and a cold rolling step of cold rolling the hot-rolled annealed steel sheet to produce a cold-rolled steel sheet.

(14) The method for manufacturing a steel sheet for hot stamping according to item (13) above, further comprising: and a plating step of optionally continuously annealing the cold-rolled steel sheet after the cold-rolling step and then plating the annealed steel sheet.

Effects of the invention

According to the present invention, a hot press-formed article having a portion with a tensile strength of less than 700MPa can be obtained, which has little variation in strength (excellent thermal stability) accompanying the coating and baking treatment.

Drawings

Fig. 1 is a schematic view showing the shape of a hot press-formed article produced in example 1.

Fig. 2 is a schematic view showing the shape of the hot press-formed article produced in example 2.

Detailed Description

The present inventors have conducted intensive studies on a method for suppressing a decrease in strength during paint baking of a hot press-formed article having a tensile strength of less than 700 MPa. As a result, the following findings were obtained.

(A) If the microstructure of the hot press-formed product contains a large amount of hard structures such as martensite and bainite, the tensile strength of the formed product is greatly reduced by the coating and baking treatment. This is considered to be because the hard tissue is tempered and softened.

(B) On the other hand, even a hot press-formed article having a metal structure mainly composed of a soft structure including ferrite with a low fraction of a hard structure may have a significantly reduced tensile strength by a coating and baking treatment depending on the chemical composition.

(C) In a hot press formed article having a metal structure mainly composed of a soft structure including ferrite, a decrease in tensile strength due to a paint bake treatment is suppressed by limiting the Mn content to a low level and containing a desired amount of Cr, and in a steel sheet before hot pressing, by controlling the Mn content and the Cr content in an iron carbide to desired amounts.

The reason is not clear, but the present inventors presume that the reason is as follows. (a) If the Mn content is excessive, the transformation temperature from austenite to ferrite decreases, and fine iron carbide or fine iron carbon clusters are generated in the ferrite in the cooling process after hot stamping, so that the ferrite is hardened. (b) The iron carbide is stabilized by controlling the Mn content and the Cr content in the iron carbide to desired amounts, and generation of fine iron carbide or fine iron carbon clusters in ferrite is suppressed. (c) Fine iron carbides or fine iron carbon clusters present in ferrite are changed into coarse iron carbides by heat treatment at the time of paint baking treatment, and the strength of ferrite is reduced.

(D) In the step of hot stamping, the temperature at which hot stamping is started is increased, thereby suppressing the decrease in tensile strength due to the coating baking treatment.

The reason is not clear, but the present inventors presume that the reason is as follows. (a) If the starting temperature of hot stamping is high, the amount of solid-solution carbon contained in ferrite in the hot-stamped product decreases. (b) Solid-solution carbon in ferrite is precipitated as coarse iron carbide by heat treatment at the time of paint baking, and the strength of ferrite is reduced.

From the above findings (a) to (D), the present inventors have found that a hot stamped product having a metal structure mainly composed of ferrite, excellent thermal stability, and a small decrease in strength due to a coating baking treatment can be produced by starting hot stamping at a desired temperature in a hot stamping step by using a steel sheet for hot stamping in which the Mn content is limited to a low level and a desired amount of Cr is contained and the Mn content and the Cr content in an iron carbide are controlled to desired amounts.

Hereinafter, the hot press-formed product (the hot press-formed product of the present embodiment) according to one embodiment of the present invention, the steel sheet for hot press (the steel sheet for hot press of the present embodiment) suitable as a material thereof, and the respective requirements of the manufacturing methods thereof will be described in detail. However, the present invention is not limited to the configurations disclosed in the present embodiment, and various modifications can be made without departing from the scope of the present invention.

< chemical composition of Hot Press molded article >

The hot press-formed product of the present embodiment has all or a part thereof having the following chemical components. The reasons for limiting the elements are as follows. In the following description, "%" as to the content of chemical components means "% by mass" as a whole. When the hot press-formed article includes a portion having a tensile strength of less than 700MPa and a portion having a tensile strength of 700MPa or more, at least the portion having a tensile strength of less than 700MPa may have the following chemical components.

C: more than 0.001% and less than 0.090%

C is an element having an effect of increasing the tensile strength of a steel sheet after hot stamping (including a steel sheet of a hot-stamped product). When the C content is less than 0.001%, the increase in tensile strength due to hot stamping cannot be expected. Therefore, the C content is set to 0.001% or more. The preferable C content is 0.010% or more, 0.020% or more, or 0.030% or more.

On the other hand, if the C content is 0.090% or more, the area ratio of martensite and/or bainite in the microstructure after hot stamping increases, and the tensile strength of the hot-stamped product becomes 700MPa or more. In this case, even if the Mn content and the Cr content are adjusted as described later, the thermal stability of the hot press-formed product cannot be ensured. Therefore, the C content is set to less than 0.090%. Preferred C content is less than 0.085%, less than 0.080%, less than 0.070%, or less than 0.060%.

Si: 2.50% or less

Si is an element contained as an impurity in steel. If the Si content exceeds 2.50%, the transformation point becomes too high while the weldability deteriorates, and it becomes difficult to heat the steel sheet to a temperature equal to or higher than the transformation point in the heating process of hot stamping. Therefore, the Si content is 2.50% or less. The Si content is preferably 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less. When a plated steel sheet is used as the steel sheet for hot stamping, the Si content is preferably less than 0.50%, more preferably less than 0.40%, and still more preferably less than 0.30% in order to ensure the plating property.

The lower limit of the Si content is not particularly limited, but an excessive decrease in the Si content causes an increase in the steel manufacturing cost. Therefore, the Si content is preferably set to 0.001%. Si also has an effect of improving the tensile strength of the steel sheet after hot stamping, and therefore can be positively contained. From the viewpoint of increasing the strength, the Si content is preferably 0.10% or more, 0.20% or more, or 0.30% or more.

Mn: more than 0.01 percent and less than 0.50 percent

Mn is an element that deteriorates the thermal stability of a hot press-formed product. In particular, if the Mn content is 0.50% or more, the thermal stability of the molded article after hot stamping is significantly deteriorated. Therefore, the Mn content is set to less than 0.50%. The Mn content is preferably less than 0.40%, less than 0.35%, less than 0.30%, less than 0.25%, or less than 0.20%.

On the other hand, Mn forms MnS by bonding with S as an impurity, and is an element having an action of suppressing embrittlement of steel due to the inclusion of S. In order to obtain this effect, the Mn content is set to 0.01% or more. The Mn content is preferably 0.05% or more, 0.10% or more, or 0.15% or more.

P: less than 0.200%

P is an element contained as an impurity in steel. If the P content exceeds 0.200%, weldability and toughness after hot stamping are significantly deteriorated, so the P content is set to 0.200% or less. The P content is preferably 0.100% or less, 0.050% or less, or 0.020% or less.

The lower limit of the P content is not particularly limited, but an excessive decrease in the P content causes an increase in the steel manufacturing cost. Therefore, the P content is preferably 0.001% or more. P also has an effect of increasing the tensile strength of the steel sheet after hot stamping, and therefore, it can be positively contained. From the viewpoint of increasing the strength, the P content is preferably 0.010% or more, 0.020% or more, or 0.030% or more. When a plated steel sheet is used as the steel sheet for hot stamping, the P content is preferably 0.050% or less, and more preferably 0.40% or less, in order to ensure the plating property.

S: 0.0200% or less

S is an element that is contained as an impurity in steel and embrittles the steel. Therefore, the smaller the S content, the more preferable, and if the S content exceeds 0.0200%, embrittlement of the steel becomes remarkable. Therefore, the S content is 0.0200% or less. The S content is preferably 0.0100% or less, 0.0050% or less, or 0.0030% or less.

The lower limit of the S content is not particularly limited, but an excessive reduction of the S content causes an increase in the steel manufacturing cost. Therefore, the S content is preferably 0.0001% or more.

sol.Al：0.001～2.500％

Al is an element having an effect of deoxidizing molten steel. If the sol.Al content (acid-soluble Al content) is less than 0.001%, deoxidation is insufficient. Therefore, the sol.al content is set to 0.001% or more. The al content is preferably 0.010% or more, 0.020% or more, or 0.040% or more.

On the other hand, if the sol.al content is too high, the transformation point rises, and it is difficult to heat the steel sheet to a temperature equal to or higher than the transformation point in the heating process of hot stamping. Therefore, the sol.al content is set to 2.500% or less. The al content is preferably 1.000% or less, 0.500% or less, 0.100% or less, or 0.060% or less.

N: 0.0200% or less

N is an element that is contained as an impurity in steel and forms a nitride in continuous casting of steel. Since the nitride deteriorates toughness after hot stamping, the N content is preferably low. If the N content exceeds 0.0200%, the deterioration of toughness becomes significant. Therefore, the N content is 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080% or less than 0.0050%.

The lower limit of the N content is not particularly limited, but since an excessive decrease in the N content causes an increase in the steel manufacturing cost, the N content is preferably 0.0010% or more.

Cr: more than 0.01 percent and less than 2.00 percent

Cr is an element having an action of improving the thermal stability of a hot-stamped product (steel sheet after hot stamping) having a metal structure mainly composed of ferrite. When the Cr content is less than 0.01%, the above-described effects cannot be sufficiently obtained even if the hot stamping start temperature in the hot stamping step is adjusted as described later. Therefore, the Cr content is set to 0.01% or more. The Cr content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

On the other hand, if the Cr content is 2.00% or more, the area ratio of martensite and/or bainite in the microstructure of the hot press-formed product becomes excessive, and the thermal stability of the hot press-formed product deteriorates. Therefore, the Cr content is set to less than 2.00%. In order to increase the yield ratio of the hot press-formed product and improve the impact absorbability, the Cr content is preferably less than 0.30%, more preferably less than 0.25%.

Ti：0～0.300％

Nb：0～0.300％

V：0～0.300％

Zr：0～0.300％

Ti, Nb, V, and Zr are elements having an effect of increasing the tensile strength of the hot press-formed product by refining the metal structure. In order to obtain this effect, 1 or more selected from Ti, Nb, V and Zr may be contained as necessary. Since these elements may not be contained, the lower limit of the content of these elements is 0%.

When the above-described effects are desired, it is preferable that 1 or more selected from Ti, Nb, V, and Zr are contained in an amount of 0.001% or more, respectively. More preferably, the alloy contains at least one of 0.005% or more of Ti, 0.005% or more of Nb, 0.010% or more of V, and 0.005% or more of Zr.

When Ti is contained, the Ti content is more preferably 0.010% or more, and particularly preferably 0.020% or more.

When Nb is contained, the Nb content is more preferably 0.020% or more, and particularly preferably 0.030% or more.

When V is contained, the V content is more preferably 0.020% or more.

When Zr is contained, the Zr content is more preferably 0.010% or more.

On the other hand, when the contents of Ti, Nb, V, and Zr respectively exceed 0.300%, the effect is saturated and the manufacturing cost of the steel sheet increases. Therefore, even when the element is contained, the content of Ti, Nb, V, and Zr is 0.300% or less.

When the contents of Ti, Nb, V, and Zr are high, carbides of these elements precipitate in a large amount, and the toughness after hot stamping may be impaired.

Therefore, the Ti content is preferably less than 0.060%, and more preferably less than 0.040%.

The Nb content is preferably less than 0.060%, and more preferably less than 0.040%.

The V content is preferably less than 0.200%, more preferably less than 0.100%.

The Zr content is preferably less than 0.200%, more preferably less than 0.100%.

Mo：0～2.00％

Cu：0～2.00％

Ni：0～2.00％

Mo, Cu, and Ni have an effect of improving the tensile strength of a hot-stamped product (a steel sheet after hot stamping). Therefore, 1 or more species selected from Mo, Cu and Ni may be contained as necessary. Since these elements may not be contained, the lower limit of the content of these elements is 0%.

When the above-described effects are desired, it is preferable that 1 or more species selected from Mo, Cu, and Ni are contained in an amount of 0.001% or more, respectively. More preferably, the content of Mo is 0.05% or more, still more preferably the content of Cu is 0.10% or more, and still more preferably the content of Ni is 0.10% or more.

On the other hand, if the contents of Mo, Cu, and Ni exceed 2.00% respectively, the area ratio of martensite and/or bainite contained in the microstructure of the hot press-formed product becomes excessive, and the thermal stability of the hot press-formed product deteriorates.

Therefore, even when the above elements are contained, the contents of Mo, Cu, and Ni are each 2.00% or less. The content of Mo is preferably 0.50% or less, the content of Cu is preferably 1.00% or less, and the content of Ni is preferably 1.00% or less.

B：0～0.0200％

B is an element that segregates in grain boundaries and has an effect of improving the toughness of the steel sheet after hot stamping. In order to obtain this effect, B may be contained as necessary. Since B may not be contained, the lower limit of the B content is 0%.

When the above-described effects are desired, the B content is preferably 0.0001% or more. The B content is more preferably 0.0006% or more, and still more preferably 0.0010% or more.

On the other hand, if the B content exceeds 0.0200%, the area ratio of martensite and/or bainite contained in the microstructure of the hot press-formed product becomes excessive, and the thermal stability of the hot press-formed product deteriorates. Therefore, even when B is contained, the B content is 0.0200% or less. The B content is preferably 0.0050% or less, and more preferably 0.0030% or less.

Ca：0～0.0100％

Mg：0～0.0100％

REM：0～0.1000％

Ca. Mg and REM are elements having an effect of improving the toughness of a steel sheet (hot-stamped steel product) after hot stamping by adjusting the shape of inclusions. Therefore, 1 or more species selected from Ca, Mg and REM may be contained as necessary. Since these elements may not be contained, the lower limit of the content of these elements is 0%.

When the above-mentioned effects are desired, it is preferable that 1 or more species selected from Ca, Mg and REM are contained in an amount of 0.0001% or more, respectively.

On the other hand, when the content of Ca or Mg exceeds 0.0100%, or when the content of REM exceeds 0.1000%, the above effect is saturated, and the production cost of the steel sheet increases. Therefore, even when the above elements are contained, the contents of Ca and Mg are 0.0100% or less and the REM content is 0.1000% or less, respectively.

In the present embodiment, REM means 17 elements in total of Sc, Y and lanthanoid, and the REM content means the total content of these elements. Lanthanum is added industrially in the form of a misch metal.

Bi：0～0.0500％

Bi is an element having an effect of improving toughness of a steel sheet (hot-stamped product) after hot stamping by refining a solidification structure. Therefore, Bi may be contained as necessary. Since Bi may not be contained, the lower limit of the Bi content is 0%.

When the above-described effects are desired, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0003% or more, and still more preferably 0.0005% or more.

On the other hand, when the Bi content exceeds 0.0500%, the above effect is saturated, and the production cost of the steel sheet increases. Therefore, even when Bi is contained, the Bi content is 0.0500% or less. The Bi content is preferably 0.0100% or less, more preferably 0.0050% or less.

The balance of the chemical components is Fe and impurities. Here, the "impurities" are components mixed in due to raw materials such as ores and scraps and various causes of the manufacturing process when industrially manufacturing a steel sheet, and refer to components that are allowed within a range that does not adversely affect the hot press-formed product of the present embodiment.

The chemical composition of the hot stamped product can be measured by a general analytical method. For example, it can be measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Al can be measured by ICP-AES using a filtrate obtained by thermally decomposing a sample with an acid. C and S can be measured by a combustion-infrared absorption method, and N can be measured by an inert gas melting-heat transfer method.

< metallic texture of Hot Press-formed article >

The metal structure (microstructure) of the hot press-formed product of the present embodiment will be explained. The hot press-formed product of the present embodiment has a metal structure including ferrite, martensite, and bainite in the amounts shown below in all or a part thereof. In the following description of the metal structure, "%" means "% by area".

Ferrite: over 60.0 percent

If the ferrite area ratio is 60.0% or less, the tensile strength of a formed article after hot stamping (hot stamped article) becomes 700MPa or more, and thermal stability cannot be ensured. Therefore, the area ratio of ferrite is set to exceed 60.0%. The ferrite area ratio is preferably more than 70.0%, more preferably more than 80.0%.

The upper limit of the area ratio of ferrite is not particularly limited, but is preferably less than 98.0%, more preferably less than 96.0%, and still more preferably less than 94.0% in order to increase the strength of the hot press-formed product.

In the present embodiment, the ferrite includes, in addition to polygonal ferrite, quasi-polygonal ferrite and granular bainitic ferrite having a higher dislocation density than the polygonal ferrite, and acicular ferrite having a jagged grain boundary.

From the viewpoint of thermal stability, the proportion of polygonal ferrite to the entire ferrite is preferably 5.0% or more in terms of area ratio.

Martensite: more than 0 percent and less than 20.0 percent

Bainite: more than 0 percent and less than 20.0 percent

If the microstructure contains a large amount of martensite and bainite, the thermal stability of the hot press-formed product is deteriorated. Therefore, the area ratios of martensite and bainite are both set to less than 20.0%. The area ratios of martensite and bainite are each preferably less than 10.0%, more preferably less than 5.0%, and still more preferably less than 2.0%.

Since martensite and bainite are not necessarily contained, the lower limit of the area ratio of martensite to bainite is 0%.

However, since martensite and bainite have an effect of increasing the strength of the hot press-formed product, they may be included in the metal structure if they are within the above ranges. If the area ratios of martensite and bainite are both less than 0.1%, the effects of the above-described effects cannot be sufficiently obtained. Therefore, when increasing the strength, the lower limit values of the area ratios of martensite and bainite are preferably 0.1% or more, and more preferably 0.5% or more.

In the present embodiment, the martensite includes a primary martensite and a tempered martensite. The primary martensite is martensite that has not been tempered, and the tempered martensite is martensite that has been self-tempered and/or tempered.

The remainder of the metal structure may contain pearlite, retained austenite, and further, precipitates such as cementite. Since it is not necessary to positively contain pearlite, retained austenite, and precipitates, the lower limit of the area ratio of pearlite, retained austenite, and precipitates is 0%.

Since pearlite has an action of increasing the strength of the hot press-formed product, when the strength is increased, the area ratio of pearlite is preferably 1.0% or more, more preferably 2.0% or more, and further preferably 5.0% or more.

On the other hand, if pearlite is excessively contained, the toughness after hot stamping is deteriorated. Therefore, the area ratio of pearlite is preferably 20.0% or less, and more preferably 10.0% or less.

The retained austenite has an effect of improving the impact absorbability of the hot press-formed article. Therefore, in order to obtain this effect, the area ratio of the retained austenite is preferably 0.5% or more, and more preferably 1.0% or more.

On the other hand, if the retained austenite is excessively contained, the toughness after hot stamping is lowered. Therefore, the area ratio of the retained austenite is preferably 5.0% or less, and more preferably 3.0% or less.

In the present embodiment, the area ratio of each metal structure is determined as follows.

First, a test piece was taken from a hot press-formed product, and after polishing a thick section of a plate (a longitudinal section of a steel plate), in the case of an unplated steel plate, texture observation was performed at a position 1/4 depth from the plate thickness of the steel plate on the surface of the steel plate (a region from 1/8 depth to 3/8 depth from the plate thickness of the surface of the steel plate), and in the case of a plated steel plate, at a position 1/4 depth from the boundary between the steel plate as a base and the plating layer (a region from 1/8 depth of the plate thickness of the steel plate as a base from the boundary to 3/8 depth of the plate thickness of the steel plate as a base from the boundary). In the case where the hot press-formed article includes a portion having a tensile strength of less than 700MPa and a portion having a tensile strength of 700MPa or more, a test piece was taken from the portion having a tensile strength of less than 700MPa and observed.

Specifically, after the investigated section of the plate thickness was subjected to nital etching or electrolytic polishing, the microstructure was observed using an optical microscope and a Scanning Electron Microscope (SEM), and the obtained microstructure photograph was subjected to image analysis, whereby the area ratios of ferrite, pearlite, bainite, and tempered martensite were obtained. Then, after LePera etching was performed at the same observation position, the structure was observed using an optical microscope and a Scanning Electron Microscope (SEM), and the total area ratio of the retained austenite and the primary martensite was calculated by image analysis of the obtained structure photograph.

In addition, for the same observation position, after the plate thickness section was electropolished, the area ratio of the retained austenite was measured using an SEM equipped with an electron back scattering pattern analyzer (EBSP).

From these results, the area ratios of ferrite, pearlite, bainite, martensite, and retained austenite were obtained.

In addition, tempered martensite can be distinguished from primary martensite by the presence of iron carbide therein, and bainite can be distinguished from bainite by the presence of iron carbide therein extending in a plurality of directions. In the present embodiment, the area ratio of martensite is obtained by calculating the area ratio of the total of the primary martensite and the tempered martensite.

< Strength of Hot Press molded article >

The hot press-formed product of the present embodiment is entirely or partially less than 700MPa in tensile strength. That is, the tensile strength of all or a part of the base steel sheet of the hot press-formed product according to the present embodiment is less than 700 MPa. This is because if the tensile strength is 700MPa or more in the entire hot press-formed product, the thermal stability of the hot press-formed product cannot be ensured. Preferably, the tensile strength is less than 600MPa or less than 560MPa in all or a part of the hot press-formed article. On the other hand, in order to improve the impact absorption of the hot press-formed article, it is preferable to set the tensile strength of the hot press-formed article to 300MPa or more, 340MPa or more, 390MPa or more, 440MPa or more, 460MPa or more, or 490MPa or more.

In order to further improve the impact absorbability of the hot press-formed article, the yield ratio (yield ratio of the steel sheet after hot press included in the hot press-formed article) is preferably 0.65 or more, more preferably 0.67 or more in a portion where the tensile strength of the hot press-formed article is less than 700 MPa. The yield ratio is determined by dividing the yield stress by the tensile strength (yield stress/tensile strength). The yield stress is an upper yield point in the case of discontinuous yield of the steel sheet after hot stamping, and is 0.2% yield strength in the case of continuous yield.

The hot press-molded article of the present embodiment may be formed by mixing a soft portion having a tensile strength of less than 700MPa and a hard portion having a tensile strength of 700MPa or more. By providing the portions having different strengths, the deformation state of the hot press-formed article at the time of collision can be controlled, and the impact absorbability of the hot press-formed article can be improved. As will be described later, the hot press-formed product having the portions different in strength can be produced by joining two or more steel sheets different in chemical composition and then hot-pressing the joined steel sheets.

< thermal stability of Hot Press molded article >

The hot press-formed article of the present embodiment has a decrease (Δ TS) in tensile strength of 100MPa or less relative to the tensile strength before heat treatment when heat treatment is performed at 170 ℃ for 20 minutes. Δ TS is preferably 60MPa or less, more preferably 30MPa or less. The lower limit of Δ TS is not particularly limited, but in order to greatly reduce Δ TS, it is necessary to excessively increase the Mn content and Cr content in iron carbide in a steel sheet for hot stamping described later, and the manufacturability of the steel sheet is impaired. Therefore, Δ TS is preferably 1MPa or more, 5MPa or more, or 10MPa or more.

The reason why the strength is reduced during the paint baking in the hot press-formed article having a structure mainly containing ferrite (area ratio exceeding 60.0%) is considered to be that: by the coating baking treatment, fine iron carbides or fine iron carbon clusters existing in the ferrite are changed into coarse iron carbides, and solid-solution carbon in the ferrite is precipitated as coarse iron carbides. It is not easy to directly and quantitatively evaluate the presence of such iron carbides, iron carbon clusters, and solid solution carbon, but it can be indirectly evaluated by the decrease amount (Δ TS) in tensile strength when heat-treated at 170 ℃ for 20 minutes. When Δ TS is 100MPa or less, generation of fine iron carbide or fine iron carbon clusters in ferrite and increase of solid solution carbon are suppressed, and it can be judged that thermal stability is excellent.

The tensile strength was obtained by collecting JIS13B tensile test pieces and subjecting the pieces to a tensile test at a tensile rate of 10 mm/min.

< coating layer >

The hot press-formed product of the present embodiment may have a plated layer on the surface. By providing the plating layer on the surface, it is possible to prevent the formation of scale during hot stamping, and further, to improve the corrosion resistance of the hot-stamped product. The kind of plating is not particularly limited as long as it is suitable for the above purpose. The hot press-formed product having a plated layer can be obtained by hot pressing using a plated steel sheet as described later. Examples of the hot press-formed article having a plated layer include hot press-formed articles having a zinc-based plated layer or an aluminum-based plated layer, which are hot pressed using a zinc-based steel sheet or an aluminum-based steel sheet, specifically, for example, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, a hot-dip aluminum-plated steel sheet, a hot-dip Zn — Al alloy-plated steel sheet, a hot-dip Zn — Al — Mg — Si alloy-plated steel sheet, an electrogalvanized steel sheet, an Ni — Zn alloy-plated steel sheet, and the like. The plating layer may be formed on one surface or both surfaces.

Next, a steel sheet for hot stamping suitable for producing the hot-stamped product will be described.

< chemical composition of Steel sheet for Hot stamping >

Since the chemical composition is not substantially changed by hot stamping, the steel sheet for hot stamping has the same chemical composition as the above-described hot stamped product.

< metallic Structure of Steel sheet for Hot stamping >

The metal structure of the steel sheet for hot stamping according to the present embodiment includes iron carbide, and the chemical composition of the iron carbide (Mn content and Cr content in the iron carbide) satisfies the following formula (i).

[Mn]_θ+[Cr]_θ>0.8…(i)

Wherein each symbol in the above formula has the following meaning.

[Mn]_θ: mn content (atomic%) in iron carbide, wherein the total content of Fe, Mn and Cr contained in iron carbide is defined as 100 atomic%

[Cr]_θ: the Cr content (atomic%) in the iron carbide when the total content of Fe, Mn and Cr contained in the iron carbide is 100 atomic%

The chemical composition of the iron carbide contained in the metal structure of the steel sheet for hot stamping satisfies the above formula (i), and thus the thermal stability of the steel sheet after hot stamping can be improved. If the left value of the above formula (i) is 0.8 or less, thermal stability cannot be ensured in the hot press-formed article even if the production conditions of the hot press-formed article are adjusted as will be described later. The value on the left side of the above formula (i) is preferably more than 1.0, more preferably more than 1.2, and further preferably more than 1.4.

On the other hand, in order to increase the Mn content and Cr content in the iron carbide, in a hot-rolled sheet annealing step described later, the hot-rolled sheet needs to be annealed at a high temperature, and the manufacturability of the steel sheet is impaired. Therefore, the left value of the above formula (i) is preferably less than 5.0, more preferably less than 4.0.

In the present embodiment, the chemical composition of iron carbide is determined by the following procedure.

First, test pieces were collected from arbitrary positions of a steel sheet, a sheet thickness cross section (vertical cross section) parallel to the rolling direction of the steel sheet was polished, and then precipitates were extracted by a replica method at a position at a depth of 1/4 mm from the sheet thickness of the steel sheet surface (region from 1/8 mm to 3/8 mm from the sheet thickness of the steel sheet surface). The precipitates were observed using a Transmission Electron Microscope (TEM), and identification and composition analysis of the precipitates were performed by electron diffraction and energy dispersive X-ray analysis (EDS).

Quantitative analysis of iron carbide by EDS was performed for 3 elements of Fe, Mn, and Cr, and the Mn content (atomic%) and the Cr content (atomic%) when the total content thereof was 100 atomic% were defined as [ Mn [, ]]_θAnd [ Cr ]]_θAnd (4) obtaining. This quantitative analysis was performed on a plurality of iron carbides, and the average value thereof was taken as the Mn content and Cr content in the iron carbide of the steel sheet. The number of iron carbides to be measured is 10 or more, and the larger the number of iron carbides to be measured, the more preferable the number of iron carbides to be measured is. Iron carbide includes cementite isolated in the metal structure, in addition to cementite constituting pearlite。

In the present embodiment, in the case of a cold-rolled steel sheet or an annealed steel sheet, the microstructure is defined at a position 1/4 depth from the thickness of the steel sheet surface (a region from 1/8 depth to 3/8 depth from the thickness of the steel sheet surface), and in the case of a plated steel sheet, at a position 1/4 depth from the boundary between the steel sheet as the base material and the plating layer (a region from 1/8 depth from the boundary to 3/8 depth from the boundary between the steel sheet as the base material and the plating layer).

The area ratio of the iron carbide is not particularly limited, but in order to refine the grain size of the metal structure after hot stamping and to improve the tensile strength, the area ratio of the iron carbide is preferably 1% or more, more preferably 3% or more.

On the other hand, if the area ratio of the iron carbide is excessive, the tensile strength of the steel sheet after hot stamping becomes too high and the thermal stability is impaired. Therefore, the area ratio of the iron carbide is preferably 20% or less, and more preferably 15% or less.

The microstructure of the steel sheet for hot stamping according to the present embodiment is mainly ferrite, but may include martensite (including primary martensite and tempered martensite), bainite, retained austenite, and precipitates other than carbide of iron as the remainder. However, since martensite, bainite, and retained austenite deteriorate toughness after hot stamping, the smaller the area ratio of these structures, the more preferable. The area ratios of martensite (including primary martensite and tempered martensite), bainite, and retained austenite are each preferably less than 1.0%, and more preferably less than 0.5%.

The area ratio of the metal structure of the steel sheet for hot stamping can be determined by the same method as in the case of the hot stamped product.

The tensile strength of the steel sheet for hot stamping is not particularly limited, but is preferably 300MPa or more or 340MPa or more from the viewpoint of the manufacturability of the steel sheet, and is preferably 650MPa or less than 590MPa from the viewpoint of the cuttability of the steel sheet.

< production method >

A preferred method for producing the hot press formed product of the present embodiment and the steel sheet for hot press of the present embodiment will be described.

[ method for producing Hot Press-molded article ]

The method for manufacturing a hot press-formed product according to the present embodiment includes: a heating step of heating a steel sheet for hot stamping having the chemical composition and the metal structure; and a hot stamping step of hot stamping the heated steel sheet for hot stamping. In the hot stamping step, cooling and forming by a die are performed to obtain a hot stamped product.

In the heating step of heating the steel sheet for hot stamping, the heating temperature T (DEG C) is set to be higher than Ac₃And (4) point. Ac of₃The point is a temperature at which ferrite disappears in the metal structure when the steel sheet material is heated, and can be determined from a thermal expansion change of the steel sheet in the heating step. If the heating temperature T (. degree. C.) is Ac₃Below this point, the formation of martensite or bainite in the microstructure of the hot press formed product is promoted, and the amount of solid-solution carbon in ferrite increases, thereby deteriorating the thermal stability of the hot press formed product. The heating temperature T (. degree. C.) is preferably (Ac)₃Point +50) DEG C or more, more preferably (Ac)₃Point +100) deg.C or higher.

Further, it is preferable that the steel sheet for hot stamping to be heated has the above-described structure.

The upper limit of the heating temperature T (. degree. C.) is not particularly limited, but if the heating temperature T (. degree. C.) is too high, the austenite becomes coarse, and the strength of the hot press-formed article is lowered. Therefore, the heating temperature T (. degree. C.) is preferably 1100 ℃ or lower, more preferably 1000 ℃ or lower, and still more preferably 950 ℃ or lower.

The holding time at the heating temperature T (. degree. C.) of the hot stamping is preferably 1 to 5 minutes.

In a hot stamping step of hot stamping a heated steel sheet for hot stamping, the start temperature of hot stamping is set to (T-80) DEG C or higher. T is the above heating temperature T (. degree. C.). If the starting temperature of hot stamping is less than (T-80). degree.C., the amount of solid-solution carbon in the ferrite in the microstructure of the hot stamped article increases, and the thermal stability of the article deteriorates. The starting temperature of hot stamping is preferably (T-50) DEG C or higher.

In order to suppress the formation of martensite or bainite in the metal structure of the hot press formed product and improve the thermal stability of the hot press formed product, the starting temperature of hot pressing is preferably (T-80) DEG C or higher and exceeds Ar₃And (4) point.

Ar₃The point is a temperature at which ferrite starts to be generated in the microstructure when the steel sheet material is cooled, and is determined from a change in thermal expansion when the steel sheet is cooled after the heating step.

Further, another method of manufacturing a hot press-formed product according to the present embodiment includes: a joining step of joining a steel sheet (steel sheet for hot stamping) having the chemical composition and the metal structure to a steel sheet for joining to produce a joined steel sheet; a heating step of heating the joined steel sheets; and a hot stamping step of hot stamping the heated joined steel sheet. Examples of the joining method include a method in which a steel sheet for hot stamping and a steel sheet for joining are butted or superposed on each other, and joined by welding.

In the hot stamping step, the heating temperature T (DEG C) of the joined steel sheet is set to exceed Ac of the steel sheet for hot stamping₃The starting temperature of hot stamping is set to (T-80) DEG C or higher. The preferable heating temperature T (. degree. C.) in this case is (Ac) of the steel sheet for hot stamping₃The point +50) DEG C or higher, and more preferably the heating temperature T (DEG C) is (Ac) of the steel sheet for hot stamping₃Point +100) deg.C or higher. The heating temperature T (. degree. C.) is preferably 1100 ℃ or lower, more preferably 1000 ℃ or lower, and still more preferably 950 ℃ or lower.

The starting temperature of hot stamping is preferably (T-50) DEG C or higher, and more preferably (T-80) DEG C or higher and exceeds Ar₃And (4) point.

The holding time at the heating temperature T (. degree. C.) of the hot stamping is preferably 1 to 5 minutes.

The reason for these is the same as that of the above-described method for producing a hot press-formed article not including the joining step.

The chemical composition and mechanical properties of the steel sheet for joining are not particularly limited. However, in order to increase the impact absorption energy of the hot-stamped product, the tensile strength of the joining steel sheet after hot stamping is preferably 700MPa or more. More preferably, the tensile strength of the joining steel sheet after hot stamping is more than 1000MPa, more than 1200MPa, or more than 1500 MPa.

In order to ensure the tensile strength of the joining steel sheet after hot stamping, the C content of the joining steel sheet is preferably 0.090% or more. More preferably 0.100% or more, 0.120% or more, or 0.200% or more. For the same reason, the Mn content of the steel sheet for joining is preferably 0.50% or more. More preferably 0.80% or more, 1.00% or more, or 1.20% or more.

The steel sheet (steel sheet for hot stamping) used as the above-described material is subjected to hot-rolled sheet annealing as described later, and is further subjected to cold rolling after the hot-rolled sheet annealing. After the cold rolling, the continuous annealing may be further performed as desired. On the other hand, as the steel sheet for joining, any of a hot-rolled steel sheet, a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet, a hot-annealed steel sheet obtained by annealing a hot-rolled steel sheet, and a cold-annealed steel sheet obtained by annealing a cold-rolled steel sheet may be used.

In order to improve the corrosion resistance of the hot press formed product, a plated steel sheet having a plated surface may be used as the hot press steel sheet and the joining steel sheet. The type of the plated steel sheet is not particularly limited, and examples thereof include hot-dip galvanized steel sheets, galvannealed steel sheets, hot-dip aluminized steel sheets, hot-dip Zn — Al alloy plated steel sheets, hot-dip Zn — Al — Mg — Si alloy plated steel sheets, electrogalvanized steel sheets, and Ni — Zn alloy plated steel sheets.

[ method for producing Steel sheet for Hot Press ]

The method for manufacturing a steel sheet for hot stamping according to the present embodiment includes: a hot rolling step of hot rolling a slab having the chemical composition and then coiling the slab at a temperature of 800 ℃ or lower to form a hot-rolled steel sheet; a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet heated to a temperature in a range exceeding 650 ℃ to obtain a hot-rolled annealed steel sheet; and a cold rolling step of cold rolling the hot-rolled annealed steel sheet.

In the hot rolling step, the coiling temperature after hot rolling is set to 800 ℃ or lower. If the coiling temperature exceeds 800 ℃ for plating, the metal structure of the hot-rolled steel sheet becomes excessively large, and the tensile strength of the steel sheet after hot stamping is reduced. The coiling temperature is preferably less than 650 ℃, more preferably less than 600 ℃, and still more preferably less than 550 ℃.

Further, if the coiling temperature is too low, the hot-rolled steel sheet is hardened and cold rolling becomes difficult, so the coiling temperature is preferably 400 ℃ or higher.

The method for producing a blank to be used in the method for producing a steel sheet for hot stamping according to the present embodiment is not particularly limited. In the exemplary preferred manufacturing method of the slab, the steel having the above-described chemical components (component compositions) is melted by a known means and then made into a steel slab by a continuous casting method, or made into a steel sheet by a cogging rolling method or the like after being made into a steel slab by an arbitrary casting method. In the continuous casting step, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to generate an external additive flow such as electromagnetic stirring of molten steel in the mold. The steel ingot or sheet may be subjected to reheating of the temporarily cooled steel ingot or sheet for hot rolling, or the steel ingot in a high-temperature state after continuous casting or the steel sheet in a high-temperature state after cogging rolling may be subjected to heat preservation directly or by auxiliary heating for hot rolling. In the present embodiment, such steel ingots and steel sheets are collectively referred to as "slabs" as hot rolling materials.

In order to prevent coarsening of austenite, the temperature of the slab for hot rolling is preferably less than 1250 ℃, and more preferably less than 1200 ℃. In order to refine the metal structure of the hot-rolled steel sheet by transforming austenite after completion of rolling, hot rolling is preferably performed at Ar₃In the temperature range above the point.

In the case where the hot rolling is composed of the rough rolling and the finish rolling, the rough rolled material may be heated between the rough rolling and the finish rolling in order to finish the finish rolling at the above temperature. In this case, it is preferable to suppress the variation in the temperature of the entire length of the rough rolled material at the start of finish rolling to 140 ℃ or less by heating the rough rolled material so that the rear end is higher in temperature than the front end. This improves the uniformity of the product characteristics in the coil after the winding step.

The heating method of the rough rolled material may be performed by a known method. For example, a solenoid type induction heating device is provided between the roughing mill and the finishing mill, and the heating temperature rise amount is controlled based on the temperature distribution in the longitudinal direction of the roughing material on the upstream side of the induction heating device.

The hot-rolled and coiled steel sheet is subjected to degreasing or other treatment as necessary by a known method, and then annealed. Annealing performed on a hot-rolled steel sheet is referred to as hot-rolled sheet annealing, and a steel sheet after the hot-rolled sheet annealing is referred to as a hot-rolled annealed steel sheet. Before the hot rolled sheet is annealed, descaling may be performed by pickling or the like.

The heating temperature in the hot-rolled sheet annealing step is more than 650 ℃. This is because the Mn content and Cr content in the iron carbide are increased in the metal structure of the hot-rolled annealed steel sheet. When the heating temperature is 650 ℃ or lower, the Mn content and Cr content in the iron carbide do not satisfy the above expression (i), and the thermal stability of the hot press-formed article cannot be ensured. The heating temperature in the hot-rolled sheet annealing step is preferably more than 680 ℃, more preferably more than 700 ℃.

On the other hand, if the heating temperature in the hot-rolled sheet annealing step is too high, the metal structure of the hot-rolled annealed steel sheet becomes coarse, and the tensile strength after hot stamping is lowered. Therefore, the heating temperature in the hot-rolled sheet annealing process is preferably less than 750 ℃, more preferably less than 720 ℃.

In order to sufficiently obtain the effect of annealing the hot-rolled sheet, it is preferable to hold the sheet at the above-mentioned heating temperature for 30 minutes or more. On the other hand, if the holding time is too long, the metal structure of the hot-rolled annealed steel sheet becomes coarse, and the tensile strength after hot stamping is lowered. Therefore, the holding time at the heating temperature in the hot-rolled sheet annealing step is preferably less than 10 hours, more preferably less than 5 hours, and still more preferably less than 2 hours.

After the hot-rolled sheet annealing step, the hot-rolled annealed steel sheet is cold-rolled to produce a cold-rolled steel sheet. This is to refine the metal structure after hot stamping and to improve the tensile strength. In order to reduce the weight of the hot-press formed product, the thickness of the cold-rolled steel sheet is preferably 2.8mm or less, more preferably 2.0mm or less, still more preferably 1.8mm or less, and particularly preferably 1.6mm or less. From the viewpoint of the manufacturability of the steel sheet, the thickness of the cold-rolled steel sheet is preferably 0.6mm or more.

The cold rolling may be performed by a conventional method, or may be performed by removing scale by pickling or the like before the cold rolling. In order to sufficiently obtain the effect of the cold rolling, the cold rolling reduction (the cumulative reduction in the cold rolling) is preferably 30% or more, and more preferably 40% or more. If the cold rolling reduction is too high, the toughness after hot stamping deteriorates, so the cold rolling reduction is preferably 65% or less, more preferably 60% or less. As will be described later, when the continuous annealing is performed after the cold rolling, the cold rolling reduction is preferably 60% or more, and more preferably 70% or more, in order to refine the metal structure of the annealed steel sheet.

After the cold rolling step, the cold-rolled steel sheet may be continuously annealed to produce an annealed steel sheet. The continuous annealing may be performed by a conventional method, or a treatment such as degreasing may be performed by a known method before the continuous annealing. In order to refine the metal structure of the annealed steel sheet by recrystallization, the soaking temperature in continuous annealing is preferably 600 ℃ or higher, 650 ℃ or higher, or 700 ℃ or higher.

On the other hand, if the heating rate in the continuous annealing is too low, the soaking temperature is too high, or the soaking time is too long, the metal structure of the annealed steel sheet is coarsened due to grain growth, and the tensile strength of the steel sheet after hot stamping is reduced, and the impact absorbability may be reduced. Therefore, the average heating rate to the soaking temperature in the continuous annealing is preferably 1 ℃/second or more, the soaking temperature is preferably 800 ℃ or less or 760 ℃ or less, and the soaking time is preferably less than 300 seconds or less than 120 seconds.

The cold-rolled steel sheet and the annealed steel sheet thus obtained may be subjected to temper rolling in accordance with a conventional method.

The steel sheet for hot stamping according to the present embodiment may have a plating layer on the surface layer thereof in order to prevent scale formation during hot stamping and to improve corrosion resistance of the steel sheet after hot stamping. The type of plating is not particularly limited as long as it is suitable for the purpose, and examples thereof include hot-dip galvanized steel sheet, galvannealed steel sheet, hot-dip aluminized steel sheet, hot-dip Zn — Al alloy-plated steel sheet, hot-dip Zn — Al — Mg — Si alloy-plated steel sheet, electrogalvanized steel sheet, and Ni — Zn alloy-plated steel sheet.

In the case of producing a hot-dip plated steel sheet, a cold-rolled steel sheet or an annealed steel sheet produced by the above-described method may be used as a base steel sheet, and the plating may be performed by a conventional method. When a cold-rolled steel sheet is used as the steel sheet material, the soaking temperature in the annealing process of the continuous hot dip plating is preferably 600 ℃ or higher, 650 ℃ or higher, or 700 ℃ or higher in order to refine the metal structure of the plated steel sheet by recrystallization.

On the other hand, if the soaking temperature is too high, it is preferable to set the soaking temperature in the annealing process of the continuous hot dip plating to 800 ℃ or lower or 760 ℃ or lower, regardless of the type of the steel sheet material, in order to coarsen the microstructure of the annealed steel sheet by grain growth. After hot dip plating, the steel sheet may be reheated to be alloyed.

In the case of manufacturing a plated steel sheet, a cold-rolled steel sheet or an annealed steel sheet manufactured by the above-described method may be used as a raw steel sheet, and after a known pretreatment for cleaning and conditioning the surface is performed as necessary, the steel sheet may be plated by a conventional method. The plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method.

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples.

[ examples ]

(example 1)

Molten steel was cast using a vacuum melting furnace to produce steels a to O having the chemical compositions shown in table 1. Ac in Table 1₁Point and Ac₃The points were determined from the thermal expansion changes when cold-rolled steel sheets of steels A to O were heated at 2 ℃/sec. In addition, Ar in Table 1₃The point is determined from the change in thermal expansion when cold-rolled steel sheets of steels A to O are cooled at 10 ℃/sec after being heated to 920 ℃.

[ Table 1]

[ Table 2]

Underlining is outside the scope of the present invention.

After heating steels A to O to 1200 ℃ and holding them for 60 minutes, hot rolling was carried out under the hot rolling conditions shown in Table 2. Specifically, in Ar₃In the above temperature range, steels A to O were rolled in 10 passes to obtain a hot-rolled steel sheet having a thickness of 3.6 mm. After hot rolling, the hot-rolled steel sheet was cooled to 540 to 580 ℃ by water spraying, the cooling completion temperature was set to a coiling temperature, the hot-rolled steel sheet was charged into an electric heating furnace kept at the coiling temperature and kept for 60 minutes, and thereafter, the hot-rolled steel sheet was furnace-cooled to room temperature at an average cooling rate of 20 ℃/hour, and slow cooling after coiling was simulated.

After the slow cooling, a hot rolled sheet annealing was performed on a part of the hot rolled steel sheet. Specifically, a hot-rolled steel sheet is heated to 620 to 710 ℃ at an average heating rate of 50 ℃/hr using an electric heating furnace, then held for 1 to 12 hours, and then cooled at an average cooling rate of 20 ℃/hr to produce a hot-rolled annealed steel sheet.

The hot-rolled steel sheet and the hot-rolled annealed steel sheet were pickled to obtain a base material for cold rolling, and cold rolling was performed at a cold rolling reduction of 61% to obtain a cold-rolled steel sheet having a thickness of 1.4 mm. A portion of the cold rolled steel sheets was heated to the soaking temperature of annealing shown in table 2 at an average heating rate of 10 c/sec using a continuous annealing simulator and soaked for 60 seconds. Subsequently, the steel sheet was cooled to 400 ℃ and held at that temperature for 180 seconds, and then cooled to room temperature to prepare an annealed steel sheet. The obtained annealed steel sheet is described as "ACR" in the column of "steel type" and "-" in the column of "plating type" in table 3. In addition, the cold-rolled steel sheet is described as "CR" in the column of "steel type" and as "-" in the column of "plating type" in table 3.

In addition, a portion of the cold rolled steel sheets was heated to the soaking temperature of annealing shown in table 2 at an average heating rate of 10 ℃/sec using a hot dip simulator and soaked for 60 seconds. Subsequently, the steel sheet is cooled and immersed in a hot-dip galvanizing bath or a hot-dip aluminizing bath to perform hot-dip galvanizing or hot-dip aluminizing, thereby producing a hot-dip galvanized steel sheet or a hot-dip aluminized steel sheet. After hot dip galvanizing, a part of the steel sheet was heated to 520 ℃ and subjected to alloying treatment to produce an alloyed hot dip galvanized steel sheet. The obtained plated steel sheet is described as "ACR" in the column of "steel type" in table 3, and is described as "GI", "GA", or "AL" in the column of "plating type".

From the cold-rolled steel sheet, annealed steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and hot-dip aluminum-plated steel sheet (these steel sheets are collectively referred to as hot-stamping steel sheets) thus obtained, test pieces for structure observation were collected and subjected to structure observation.

Specifically, in the case of non-plated steel sheets (cold rolled steel sheets and annealed steel sheets), precipitates were extracted by a replica method at a position 1/4 depth from the thickness of the steel sheet surface (a region from 1/8 depth to 3/8 depth from the thickness of the steel sheet surface) after polishing a section of the steel sheet parallel to the rolling direction, and in the case of plated steel sheets, at a position 1/4 depth from the boundary between the steel sheet of the substrate and the plating layer (a region from 1/8 depth to 3/8 depth from the boundary between the steel sheet of the substrate and the thickness of the steel sheet of the substrate), and iron carbides were identified by TEM. For 10 iron carbides, 3 elements of Fe, Mn, and Cr were quantitatively analyzed using EDS. The Mn content (atomic%) and the Cr content (atomic%) in the iron carbide when the total content of Fe, Mn and Cr is 100 atomic% are defined as [ Mn%]_θAnd [ Cr ]]_θTo obtain [ Mn]_θAnd [ Cr ]]_θAverage of the sum of (a) and (b).

Tensile test pieces of JIS13B were collected from the hot stamping steel sheet in a direction perpendicular to the rolling direction, and tensile test was performed at a tensile rate of 10 mm/min to determine tensile strength. Table 3 shows the results of observing the metal structure of the steel sheet for hot stamping and the results of examining the mechanical properties of the steel sheet for hot stamping.

[ Table 3]

Underlining is outside the scope of the present invention.

#1 CR: cold-rolled steel sheet, ACR: annealing the steel plate, and annealing the steel plate,

#2 GI: hot-dip galvanized steel sheet, GA: an alloyed hot-dip galvanized steel sheet, which is,

AL: hot-dip aluminized steel sheet, non-plated steel sheet

A hot stamping steel sheet having a width of 240mm and a length of 170mm was taken out from the hot stamping steel sheet, and a cap member having a shape shown in FIG. 1 was produced by hot stamping. In the hot stamping step, the raw material plate was heated at the heating temperature shown in table 4 for 4 minutes using a gas heating furnace, then taken out of the heating furnace, left to cool, and capped at the starting temperature shown in table 4 with a mold equipped with a cooling device interposed therebetween.

A part of the cap member (hot press-formed article) was subjected to a heat treatment at 170 ℃ for 20 minutes of a coating and baking treatment using an electric heating furnace.

Test pieces for SEM observation were collected from the stamped bottom (パンチ bottom) of the cap member before heat treatment, the test piece was polished on a section parallel to the rolling direction of the steel sheet, and then subjected to nital etching and LePera etching, and in the case of a non-plated steel sheet, when observing the microstructure at a position 1/4 depth from the plate thickness on the steel sheet surface (region 1/8 depth from the plate thickness on the steel sheet surface to 3/8 depth from the plate thickness on the steel sheet surface), in the case of a plated steel sheet, the microstructure was observed at a position 1/4 depth from the boundary between the steel sheet as the base and the plating layer (region from 1/8 depth of the thickness of the steel sheet as the base from the boundary to 3/8 depth of the thickness of the steel sheet as the base from the boundary). The area ratios of ferrite, martensite, and bainite were measured by image processing using the above method. The results are shown in Table 4. In table 4, in the test numbers satisfying the specification of the present invention, the ratio of polygonal ferrite to ferrite in the microstructure of the hot press formed product was 5.0% or more.

Tensile test pieces of JIS No. 13B were collected from the punched bottom of the cap member before heat treatment along the longitudinal direction of the member, and tensile test was carried out at a tensile rate of 10 mm/min to determine tensile strength, yield stress and yield ratio. The yield stress is the upper yield point in the case of discontinuous yield and 0.2% yield strength in the case of continuous yield.

Further, tensile test pieces of JIS13B were similarly collected from the press bottom of the cap member after heat treatment, and tensile test was similarly performed to determine tensile strength. The difference (Δ TS) between the tensile strength of the cap member not subjected to the heat treatment and the tensile strength of the cap member subjected to the heat treatment was obtained, and if Δ TS is 100MPa or less, the cap member was judged to have good thermal stability.

The tensile strength before heat treatment was less than 700MPa and Δ TS was 100MPa or less, and the specimen was judged to be acceptable when the present invention was satisfied. When the yield ratio before the heat treatment was 0.65 or more, it was judged that the impact absorption ability was more excellent.

Table 4 shows the results of observation of the metal structure of the cap member and the results of evaluation of the mechanical properties of the cap member. In table 4, underlined values are meant to be outside the scope of the present invention.

[ Table 4]

The hot press-formed articles of test numbers 1, 3, 4, 6, 9, 11, 14, 16, 19, 22 to 27, 31 and 33 satisfying the specification of the present invention all had tensile strengths of less than 700MPa and Δ TS of 100MPa or less, and showed good thermal stability.

The yield ratios of all the hot stamped products of test nos. 1, 3, 4, 6, 9, 11, 14, 16, 22 to 27, 31 and 33 having a Cr content of less than 0.30% were 0.65 or more for the chemical components of the hot stamped products, and the strength characteristics were particularly good.

In the hot-rolled sheet annealing step, the tensile strength of the hot-press-formed articles of test nos. 1, 3, 4, 9, 11, 14, 16, 22 to 27, 31 and 33, which had a holding time at a heating temperature of less than 10 hours, was 440MPa or more, and the strength characteristics were particularly good.

On the other hand, the hot press molded articles of test nos. 20, 21 and 28 of comparative examples having chemical compositions outside the range of the present invention had tensile strengths of 700MPa or more and Δ TS of 100MPa or more, or Δ TS of 100MPa or more, and thermal stability was poor.

Specifically, in test No. 20 using steel E, Δ TS was large because the Mn content of the steel was too high.

In test No. 21 using steel F, the C content of the steel was too high, and therefore the area ratio of ferrite was insufficient and the area ratios of martensite and bainite were excessive in the microstructure of the hot press-formed product, and the tensile strength (before heat treatment) of the hot press-formed product was 700MPa or more, and Δ TS was large.

In test No. 28 using steel M, since the Cr content of the steel was too high, the area ratio of ferrite was insufficient and the area ratio of martensite was excessive in the microstructure of the hot press-formed product, and the tensile strength (before heat treatment) of the hot press-formed product was 700MPa or more, and Δ TS was large.

The chemical components are within the range of the present invention, but the metal structure of the steel sheet for hot stamping or the method of producing the hot stamped product deviates from the range of the present invention, and the hot stamped product has a Δ TS of 100MPa or more and is inferior in thermal stability in test nos. 2, 5, 7, 8, 10, 12, 13, 15, 17, 18, 29, 30 and 32 of the comparative examples.

Specifically, test No. 8 using steel a, test No. 13 using steel B, test No. 18 using steel C, test No. 29 using steel a, and test No. 30 using steel B were not subjected to hot-rolled sheet annealing, and therefore the sum of the Mn content and the Cr content in the iron carbide in the microstructure of the steel sheet for hot stamping was low, and Δ TS was large in the hot-stamped steel product.

In test No. 7 using steel a, the heating temperature in the hot-rolled sheet annealing step was too low, so that the sum of the Mn content and the Cr content in the iron carbide in the metal structure of the steel sheet for hot stamping was low, and Δ TS was large in the hot-stamped steel product.

Test No. 2 using steel a, test No. 10 using steel B, test No. 15 using steel C, and test No. 32 using steel N, since the hot press start temperature in the hot press step was too low, Δ TS was large in the hot press-formed product.

In test No. 5 using steel a, test No. 12 using steel B, and test No. 17 using steel C, since the heating temperature in the heating step was too low, Δ TS was large in the hot press-formed product.

(example 2)

In example 1, molten steel was cast using a vacuum melting furnace, and steels a to C having chemical compositions shown in table 1 were manufactured. Hot rolling, hot-rolled sheet annealing, cold rolling, and annealing were performed under the conditions shown in table 5 using steels a to C in the same manner as in example 1, followed by plating treatment, to produce a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a hot-dip aluminum-plated steel sheet (steel sheet for hot stamping).

[ Table 5]

The microstructure and mechanical properties of these hot stamping steel sheets were examined in the same manner as in example 1. Table 6 shows the results of observing the metal structure of the steel sheet for hot stamping and the results of examining the mechanical properties of the steel sheet for hot stamping.

[ Table 6]

#3 ACR: annealed steel sheet

#4 GI: hot-dip galvanized steel sheet, GA: an alloyed hot-dip galvanized steel sheet, which is,

AL: hot-dip aluminized steel sheet

From these hot stamping steel sheets, a hot stamping raw material sheet having a thickness of 1.4mm, a width of 240mm and a length of 170mm was collected. These raw material plates were joined to a joining steel plate of the same size by laser welding to prepare a joining steel plate having a thickness of 1.4mm, a width of 240mm and a length of 340 mm. The steel sheet for joining used a cold rolled steel sheet having a chemical composition of, by mass%, 0.21% C-0.13% Si-1.31% Mn-0.012% P-0.0018% S-0.043% sol.Al-0.0030% N-0.21% Cr-0.0018% B.

A cap member having a shape shown in fig. 2 was produced by hot-pressing a joined steel sheet under the conditions shown in table 7, similarly to example 1. Thereafter, a part of the cap member was subjected to a heat treatment at 170 ℃ for 20 minutes equivalent to a paint bake treatment using an electric heating furnace.

Then, with respect to the cap members before and after the heat treatment, the metal structure and the mechanical properties of the portions composed of steels a to C were examined in the same manner as in example 1. Table 7 shows the results of observation of the metal structure of the cap member (hot press-formed article) and the results of evaluation of the mechanical properties of the cap member.

[ Table 7]

The results of any of test numbers 34 to 36 showed good thermal stability, in which the TS (before heat treatment) of the hot press-formed article was less than 700MPa and Δ TS was 100MPa or less. The metal structure of the joining steel plate portion of the cap member was a martensite single structure, and the tensile strengths thereof were 1580MPa, 1583MPa, and 1575MPa in test Nos. 34 to 36, respectively.

Industrial applicability

According to the present invention, a hot-stamped product having excellent thermal stability in a portion where the variation in strength accompanying the coating and baking treatment is small and the tensile strength is less than 700MPa, a steel sheet for hot stamping suitable as a material thereof, and a method for producing the same can be obtained.