KR20230137436A - Steel sheets for hot stamping and hot stamping molded bodies - Google Patents

Abstract

The steel sheet for hot stamping of the present invention has a predetermined chemical composition, S _α + S _GB , which is the sum of the area ratio S _α of ferrite and the area ratio S _GB of granular bainite, is 10% or more and less than 50%, and the granular It has a metal structure in which _SGB / _Sα , _which is the ratio of the area ratio _SGB of rubinite and the area ratio Sα of ferrite, is 0.30 to 0.70. In addition, the hot stamp molded body manufactured by the steel sheet for hot stamping of the present invention has a predetermined chemical composition, the average particle size of the old austenite particles is 5 to 25 μm, and the standard deviation of the particle size distribution of the old austenite particles is 0.1. It has a metal structure of 2.0 ㎛ to 2.0 ㎛ and a tensile strength of 2200 MPa or more.

Description

Steel sheets for hot stamping and hot stamping molded bodies

The present invention relates to a steel sheet for hot stamping and a hot stamping molded body.

This application claims priority based on Patent Application No. 2021-081620 filed in Japan on May 13, 2021, and uses the content here.

Conventionally, from the viewpoint of global environmental issues and crash safety performance, there has been a demand for thinner and higher strength automobile members. To meet these demands, the number of automobile components made of high-strength steel sheets is increasing. Additionally, as a method for forming high-strength steel sheets, a method called hot stamping is known. In hot stamping, a high-strength steel sheet is press-formed at a high temperature of 700°C or higher, and quenching is performed inside a press mold or outside the press mold. According to hot stamping, forming is performed in a high temperature range where the strength of the steel sheet decreases, so forming defects that occur in cold pressing can be suppressed. In addition, since a structure with martensite as the main phase is obtained by quenching after molding, high strength can be obtained. For this reason, hot stamp molded bodies with a tensile strength of about 1500 MPa are widely used worldwide.

In an automobile member formed by hot stamping a high-strength steel plate, in order to obtain a greater effect of reducing the weight of the automobile body, it is necessary to obtain a member that has high strength and also has excellent crash characteristics. In order to improve the crash characteristics of automobile members, automobile members are particularly required to have excellent bending properties.

Patent Document 1 discloses a hot stamped molded body that has a tensile strength of 1900 MPa or more and can suppress low-stress fracture, and a method for manufacturing the same.

The present inventors found that in automobile members with further improved tensile strength, it is necessary to further improve bendability in order to obtain a higher car body weight reduction effect.

International Publication No. 2018/134874

Acta Materialia, 58(2010), 6393-6403

The present invention has been made in view of the above problems. The purpose of the present invention is to provide a hot stamp molded body with high strength and excellent bendability, and a steel plate for hot stamping from which this hot stamp molded body can be manufactured.

The gist of the present invention is as follows.

[1] The steel sheet for hot stamping according to one aspect of the present invention has a chemical composition in mass%,

C: greater than 0.40%, less than or equal to 0.70%,

Si: 0.010 to 1.30%,

Mn: more than 0.60%, less than 3.00%,

P: 0.100% or less,

S: 0.0100% or less,

N: 0.0130% or less,

O: 0.0200% or less,

Al: 0.0010 to 0.500%,

Cr: 0.010 to 0.80%,

Nb: 0 to 0.100%,

Ti: 0 to 0.100%,

B: 0 to 0.0100%,

Mo: 0 to 1.00%,

Co: 0 to 2.00%,

Ni: 0% or more, less than 3.00%,

Cu: 0 to 1.00%,

V: 0 to 1.00%,

W: 0 to 1.000%,

Ca: 0 to 0.010%,

Mg: 0 to 1.000%,

REM: 0 to 1.000%,

Sb: 0 to 1.000%,

Zr: 0 to 1.000%,

Sn: 0 to 1.000%, and

As: 0 to 0.100%

Contains, the balance consists of Fe and impurities,

S _α + S GB _, which is the sum of the area ratio S _α of ferrite and the area ratio S _GB of granular bainite, is 10% or more and less than 50%,

It has a metal structure in which S _GB /S _α , which is the ratio of the area ratio S _GB of the granular bainite and the area ratio S _α of the ferrite, is 0.30 to 0.70.

[2] The steel sheet for hot stamping described in [1] above has the above chemical composition in mass%,

Nb: 0.001 to 0.100%,

Ti: 0.010 to 0.100%,

B: 0.0015 to 0.0100%,

Mo: 0.05 to 1.00%,

Co: 0.05 to 2.00%,

Ni: 0.01% or more, less than 3.00%,

Cu: 0.01 to 1.00%,

V: 0.01 to 1.00%,

W: 0.001 to 1.000%,

Ca: 0.001 to 0.010%,

Mg: 0.001 to 1.000%,

REM: 0.001 to 1.000%,

Sb: 0.005 to 1.000% and

Zr: 0.001 to 1.000%,

Sn: 0.001 to 1.000%, and

As: 0.001 to 0.100%

It may contain one or two or more types selected from the group consisting of.

[3] The hot stamp molded body according to another aspect of the present invention has a chemical composition in mass%,

C: greater than 0.40%, less than or equal to 0.70%,

Si: 0.010 to 1.30%,

Mn: more than 0.60%, less than 3.00%,

P: 0.100% or less,

S: 0.0100% or less,

N: 0.0130% or less,

O: 0.0200% or less,

Al: 0.0010 to 0.500%,

Cr: 0.010 to 0.80%,

Nb: 0 to 0.100%,

Ti: 0 to 0.100%,

B: 0 to 0.0100%,

Mo: 0 to 1.00%,

Co: 0 to 2.00%,

Ni: 0% or more, less than 3.00%,

Cu: 0 to 1.00%,

V: 0 to 1.00%,

W: 0 to 1.000%,

Ca: 0 to 0.010%,

Mg: 0 to 1.000%,

REM: 0 to 1.000%,

Sb: 0 to 1.000%,

Zr: 0 to 1.000%,

Sn: 0 to 1.000%, and

As: 0 to 0.100%

Contains, the balance consists of Fe and impurities,

It has a metal structure in which the average particle size of the old austenite particles is 5 to 25 ㎛, and the standard deviation of the particle size of the old austenite particles is 0.1 to 2.0 ㎛,

The tensile strength is more than 2200 MPa.

[4] The hot stamp molded body described in [3] above has the chemical composition in mass%,

Nb: 0.001 to 0.100%,

Ti: 0.010 to 0.100%,

B: 0.0015 to 0.0100%,

Mo: 0.05 to 1.00%,

Co: 0.05 to 2.00%,

Ni: 0.01% or more, less than 3.00%,

Cu: 0.01 to 1.00%,

V: 0.01 to 1.00%,

W: 0.001 to 1.000%,

Ca: 0.001 to 0.010%,

Mg: 0.001 to 1.000%,

REM: 0.001 to 1.000%,

Sb: 0.005 to 1.000%,

Zr: 0.001 to 1.000%,

Sn: 0.001 to 1.000%, and

As: 0.001 to 0.100%

It may contain one or two or more types selected from the group consisting of.

[5] In the hot stamp molded body according to [3] or [4] above, the area ratio of the prior austenite particles having an average particle diameter of 0.5 to 3.0 μm may be 60% or less.

According to the above aspect of the present invention, a hot stamp molded body having high strength and excellent bendability, and a steel sheet for hot stamping from which this hot stamp molded body can be manufactured can be provided.

The present inventors studied the bendability of hot stamped molded bodies. As a result, the present inventors found that the bendability deteriorates when a large amount of fine old austenite particles are present in the metal structure of a hot stamped body. In addition, the present inventors, in the metal structure of a hot stamp molded body, set the old austenite particles to a desired size and suppress the variation in the size of the old austenite particles, that is, by sizing the old austenite particles, It was found that the bendability of the molded body could be further improved.

Next, the present inventors studied a method of obtaining the hot stamp molded body. As a result, the present inventors created a desired amount of ferrite and granular bainite in the metal structure of a steel sheet for hot stamping, and also adjusted the area ratio of ferrite and the area ratio of granular bainite to have a desired relationship. It was found that by controlling, the hot stamp molded body can be obtained.

Below, a steel sheet for hot stamping and a hot stamping molded body according to the present embodiment made based on the above findings will be described. First, the reason for limiting the chemical composition of the hot stamping steel sheet according to this embodiment will be explained.

In addition, the numerical limitation range described below with "to" in between includes the lower limit and the upper limit. Numerical values indicated as “less than” or “exceeding” are not included in the numerical range. % for chemical composition all indicate mass %.

The steel sheet for hot stamping according to the present embodiment has a chemical composition in mass%: C: more than 0.40%, 0.70% or less, Si: 0.010 to 1.30%, Mn: more than 0.60%, 3.00% or less, P: 0.100%. Hereinafter, S: 0.0100% or less, N: 0.0130% or less, O: 0.0200% or less, Al: 0.0010 to 0.500%, Cr: 0.010 to 0.80%, and the remainder consists of Fe and impurities. Hereinafter, each element will be explained.

C: More than 0.40%, less than 0.70%

C greatly contributes to improving the strength of the hot stamped molded body. If the C content is 0.40% or less, it becomes difficult to obtain sufficient strength in the hot stamp molded body. Therefore, the C content is set to exceed 0.40%. Preferably it is 0.42% or more, more preferably 0.45% or more, and even more preferably 0.47% or more.

On the other hand, if the C content is more than 0.70%, coarse carbides are generated and the bendability of the hot stamped body is deteriorated. Therefore, the C content is set to 0.70% or less. Preferably it is 0.65% or less, and more preferably 0.60% or less.

Si: 0.010 to 1.30%

Si is an element that improves the deformability of hot stamped molded products by suppressing the formation of oxides that combine with oxygen and become the starting point of destruction. If the Si content is less than 0.010%, coarse oxides are formed in the hot stamp molded body, and the desired bendability cannot be obtained. Therefore, the Si content is set to 0.010% or more. Preferably it is 0.05% or more, and more preferably 0.10% or more.

On the other hand, if the Si content is more than 1.30%, coarse oxides are generated and the bendability of the hot stamp molded body deteriorates. Therefore, the Si content is set to 1.30% or less. Preferably it is less than 1.00%, more preferably 0.50% or less.

Mn: more than 0.60%, less than 3.00%

Mn stabilizes austenite and improves the hardness of the steel sheet. If the Mn content is 0.60% or less, sufficient hardening properties cannot be obtained. Therefore, the Mn content is set to exceed 0.60%. Preferably it is 0.80% or more, and more preferably 1.20% or more.

On the other hand, if the Mn content exceeds 3.00%, coarse inclusions are generated and the bendability of the hot stamp molded body is deteriorated. Therefore, the Mn content is set to 3.00% or less. Preferably it is 2.20% or less, and more preferably 1.80% or less.

P: 0.100% or less

P segregates at the grain boundaries of the steel sheet and deteriorates the bendability of the hot stamped body. Therefore, the lower the P content, the more preferable it is. In particular, if the P content exceeds 0.100%, the processability of the steel sheet and the bendability of the hot stamped body are significantly deteriorated. Therefore, the P content is set to 0.100% or less. Preferably it is 0.080% or less, and more preferably 0.020% or less.

The lower limit of the P content is not particularly limited, but may be 0%. However, if the P content is reduced to less than 0.0001%, the cost of P removal increases significantly, making it economically undesirable. Therefore, the P content may be 0.0001% or more.

S: 0.0100% or less

S forms coarse inclusions and deteriorates the bendability of the hot stamp molded body. For this reason, the lower the S content, the more preferable. In particular, if the S content exceeds 0.0100%, the formability of the steel sheet and the bendability of the hot stamped body are significantly deteriorated. Therefore, the S content is set to 0.0100% or less. Preferably it is 0.0050% or less, and more preferably 0.0010% or less.

The lower limit of the S content is not particularly limited, but may be 0%. However, if the S content is reduced to less than 0.0001%, the cost of S removal increases significantly, which is economically undesirable. Therefore, the S content may be 0.0001% or more.

N: 0.0130% or less

N forms coarse nitrides and deteriorates the bendability of the hot stamped molded body. For this reason, the lower the N content, the more preferable it is. In particular, when the N content exceeds 0.0130%, the formability of the steel sheet is significantly deteriorated. Therefore, the N content is set to 0.0130% or less. Preferably it is 0.0100% or less or 0.0070% or less, and more preferably 0.0040% or less.

The lower limit of the N content is not particularly limited, but may be 0%. However, if the N content is reduced to less than 0.0001%, the cost of N removal increases significantly, which is economically undesirable. Therefore, the N content may be 0.0001% or more.

O: 0.0200% or less

O forms coarse oxides in the steel and deteriorates the bendability of the hot stamped body. For this reason, the lower the O content, the more preferable it is. In particular, when the O content exceeds 0.0200%, the bendability of the hot stamp molded body is significantly deteriorated. Therefore, the O content is set to 0.0200% or less. Preferably it is 0.0100% or less, and more preferably 0.0060% or less.

The lower limit of the O content is not particularly limited, but may be 0%. However, if the O content is reduced to less than 0.0001%, the manufacturing cost increases significantly, which is economically undesirable. Therefore, the O content may be 0.0001% or more.

Al: 0.0010 to 0.500%

Al is an element that deoxidizes molten steel and suppresses the formation of oxides, which are the origins of destruction, thereby improving deformability and improving the bendability of hot stamped molded bodies. If the Al content is less than 0.0010%, deoxidation is not sufficiently performed, coarse oxides are generated, and the above effect cannot be obtained. Therefore, the Al content is set to 0.0010% or more. Preferably it is 0.010% or more, and more preferably 0.030% or more.

On the other hand, if the Al content exceeds 0.500%, coarse oxides are generated in the steel, and the bendability of the hot stamped product decreases. Therefore, the Al content is set to 0.500% or less. Preferably it is 0.450% or less, and more preferably 0.350% or less.

Cr: 0.010 to 0.80%

Cr increases the strength of the hot stamped body by being dissolved in prior austenite particles during heating during hot stamping. If the Cr content is less than 0.010%, this effect cannot be obtained. Therefore, the Cr content is set to 0.010% or more. Preferably it is 0.10% or more, and more preferably 0.20% or more.

On the other hand, if the Cr content exceeds 0.80%, coarse carbides are formed and the bendability of the hot stamped body is deteriorated. Therefore, the Cr content is set to 0.80% or less. Preferably it is 0.60% or less, more preferably 0.40% or less.

The remainder of the chemical composition of the hot stamping steel sheet according to this embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed from steel raw materials or scrap and/or during the steelmaking process, or elements that are allowed as long as they do not impair the characteristics of the hot stamp molded body according to the present embodiment.

The steel sheet for hot stamping according to this embodiment may contain the following elements as arbitrary elements instead of part of Fe. When the following arbitrary elements are not contained, the content is 0%.

Nb: 0 to 0.100%

Nb forms carbonitride in the steel and improves the strength of the hot stamped body through precipitation strengthening. In order to obtain this effect, the Nb content is preferably 0.001% or more.

On the other hand, if the Nb content is more than 0.100%, a large amount of carbonitride is generated in the steel, and the bendability of the hot stamped product is reduced. Therefore, the Nb content is set to 0.100% or less.

Ti: 0 to 0.100%

Ti, like Nb, forms carbonitride in steel and improves the strength of the hot stamped body through precipitation strengthening. In order to obtain this effect, the Ti content is preferably 0.010% or more.

On the other hand, if the Ti content exceeds 0.100%, a large amount of carbonitride is generated in the steel, and the bendability of the hot stamped product is reduced. Therefore, the Ti content is set to 0.100% or less.

B: 0 to 0.0100%

B improves the hardness of the steel and improves the strength of the hot stamped body. In order to obtain this effect, the B content is preferably set to 0.0015% or more.

On the other hand, if the B content exceeds 0.0100%, coarse carbides are generated and the bendability of the hot stamped body is deteriorated. Therefore, the B content is set to 0.0100% or less.

Mo: 0 to 1.00%

Mo improves the hardening properties of the steel sheet and improves the strength of the hot stamped body. In order to obtain this effect, it is preferable that the Mo content is 0.05% or more.

On the other hand, if the Mo content exceeds 1.00%, coarse carbides are generated and the bendability of the hot stamped body is deteriorated. Therefore, the Mo content is set to 1.00% or less.

Co: 0 to 2.00%

Co improves the hardness of the steel sheet and improves the strength of the hot stamped body. In order to reliably exhibit this effect, it is preferable that the Co content is 0.05% or more.

On the other hand, if the Co content exceeds 2.00%, coarse carbides are generated and the bendability of the hot stamped body deteriorates. Therefore, the Co content is set to 2.00% or less.

Ni: 0% or more, less than 3.00%

Ni improves the hardening properties of the steel sheet and improves the strength of the hot stamped body. In order to obtain this effect, the Ni content is preferably 0.01% or more.

On the other hand, if the Ni content is 3.00% or more, segregation is promoted and the bendability of the hot stamped product is deteriorated. Therefore, the Ni content is set to less than 3.00%.

Cu: 0 to 1.00%

Cu, like Ni, improves the hardness of the steel sheet and improves the strength of the hot stamped body. In order to obtain this effect, the Cu content is preferably 0.01% or more.

On the other hand, if the Cu content exceeds 1.00%, segregation is promoted and the bendability of the hot stamped product is deteriorated. Therefore, the Cu content is set to 1.00% or less.

V: 0 to 1.00%

V improves the hardness of the steel sheet and improves the strength of the hot stamped body. In order to achieve this effect, the V content is preferably 0.01% or more.

On the other hand, if the V content exceeds 1.00%, coarse carbides are generated and the bendability of the hot stamped body is deteriorated. Therefore, the V content is set to 1.00% or less.

W: 0 to 1.000%

W improves the hardness of the steel sheet and improves the strength of the hot stamped body. In order to obtain this effect, the W content is preferably set to 0.001% or more.

On the other hand, if the W content exceeds 1.000%, segregation is promoted and the bendability of the hot stamped product deteriorates. Therefore, the W content is set to 1.000% or less.

Ca: 0 to 0.010%

Ca improves the deformability by suppressing the formation of oxides, which are the origins of destruction, and increases the bendability of the hot stamped molded body. In order to reliably obtain this effect, it is preferable that the Ca content is 0.001% or more.

On the other hand, if the Ca content is more than 0.010%, coarse oxides are generated and the bendability of the hot stamp molded body is deteriorated. Therefore, the Ca content is set to 0.010% or less.

Mg: 0 to 1.000%

Mg improves the deformability by suppressing the formation of oxides, which are the origins of destruction, and increases the bendability of the hot stamped molded body. In order to obtain this effect, the Mg content is preferably 0.001% or more.

On the other hand, if the Mg content exceeds 1.000%, coarse oxides are generated and the bendability of the hot stamp molded body is deteriorated. Therefore, the Mg content is set to 1.000% or less.

REM: 0 to 1.000%

REM improves deformability by suppressing the formation of oxides, which are the starting points of destruction, and increases the bendability of hot stamped molded products. In order to obtain this effect, it is preferable that the REM content is 0.001% or more.

On the other hand, if the REM content exceeds 1.000%, coarse oxides are generated and the bendability of the hot stamp molded body deteriorates. Therefore, the REM content is set to 1.000% or less.

In addition, in this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoid, and the content of REM refers to the total content of these elements.

Sb: 0 to 1.000%

Sb improves the deformability by suppressing the formation of oxides, which are the origins of destruction, and increases the bendability of the hot stamped molded body. In order to obtain this effect, it is preferable that the Sb content is 0.005% or more.

On the other hand, if the Sb content exceeds 1.000%, coarse oxides are generated and the bendability of the hot stamp molded body deteriorates. Therefore, the Sb content is set to 1.000% or less.

Zr: 0 to 1.000%

Zr improves the deformability by suppressing the formation of oxides, which are the origins of destruction, and increases the bendability of the hot stamped molded body. In order to obtain this effect, it is preferable that the Zr content is 0.001% or more.

On the other hand, if the Zr content exceeds 1.000%, coarse oxides are generated and the bendability of the hot stamped product deteriorates. Therefore, the Zr content is set to 1.000% or less.

Sn: 0 to 1.000%

Sn improves the deformability by suppressing the formation of oxides, which are the origins of destruction, and increases the bendability of the hot stamped molded body. In order to reliably obtain this effect, it is preferable that the Sn content is 0.001% or more.

On the other hand, since the above effect is saturated even if it is contained in a large amount, the Sn content is set to 1.000% or less.

As: 0 to 0.100%

As lowers the austenite single phase temperature, refines the old austenite particles and increases the bendability of the hot stamped body. In order to reliably obtain this effect, the As content is preferably set to 0.001% or more.

On the other hand, since the above effect is saturated even if it is contained in a large amount, the As content is set to 0.100% or less.

The chemical composition of the steel sheet for hot stamping described above may be measured by a general analysis method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Additionally, C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. When the steel sheet for hot stamping has a plating layer on the surface, the plating layer can be removed by mechanical grinding and then the chemical composition can be analyzed.

Next, the metal structure of the hot stamping steel sheet according to this embodiment will be described.

In the steel sheet for hot stamping according to the present embodiment, S _α + S _GB , which is the sum of the area ratio S _α of ferrite and the area ratio S _GB of granular bainite, is 10% or more and less than 50%, and the granular bainite It has a metal structure in which _SGB / _Sα , which is the ratio of the area ratio _SGB and the area ratio _Sα of the ferrite, is 0.30 to 0.70. Below, each regulation is explained.

In addition, in this embodiment, the position of the sheet thickness cross section parallel to the rolling direction at a depth of 1/4 of the sheet thickness from the surface (area from a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) Specifies the metal structure in . The reason is that the metal structure at this position represents the typical metal structure of a steel plate.

“S _α + S GB, which _is the sum of the area ratio S _α of ferrite and the area ratio S _GB of granular bainite, is 10% or more and less than 50%.”

If S _α + S _GB , which is the sum of the area ratio S _α of ferrite and the area ratio S _GB of granular bainite, is less than 10%, prior austenite particles cannot be sized in the hot stamped molded body, and as a result, bendability decreases. This excellent hot stamped molded body cannot be obtained. Since the solid solution limit of carbon in ferrite and granular bainite is low, by setting S _α +S _GB to 10% or more and S _GB /S _α , described later, within the desired range, carbon diffuses into the ferrite grain boundaries, forming ferrite. A carbon segregation region is formed at the grain boundary. During hot stamping, the carbon segregation region becomes the starting point for old austenite particles, so that old austenite particles are uniformly dispersed and generated. As a result, it is assumed that old austenite particles can be sized in a hot stamped molded body. S _α + S _GB is preferably 20% or more, and more preferably 30% or more.

On the other hand, when S _α + S _GB is 50% or more, the segregation of carbon into the ferrite grain boundaries is promoted too much, and the production density of carbides at the ferrite grain boundaries increases, causing the old austenite particles to be uniformly dispersed after hot stamping. Can not. S _α + S _GB is preferably 40% or less.

“S _GB /S _α , the ratio of the area ratio S _GB of granular bainite and the area ratio S _α of ferrite, is 0.30 to 0.70.”

S _GB /S _α is set to 0.30 to 0.70. Since ferrite does not contain subgrain boundaries, carbon is less likely to segregate within the grains than granular bainite. By controlling the area ratio of ferrite and granular bainite to the above range, the amount of carbon segregation at ferrite grain boundaries can be increased. Since the subgrain boundaries contained in the crystal grains of granular bainite can become segregation origins of carbon, they function as origins of prior austenite during hot stamping heating. As a result, the average particle size of the prior austenite particles in the hot stamp molded body can be controlled to 25 μm or less. S _GB /S _α is preferably 0.40 or more.

On the other hand, when S _GB /S _α exceeds 0.70, segregation of carbon into subgrain boundaries is promoted too much, and the adjacent distance of austenite grains becomes closer during hot stamp heating, so that the average grain size of prior austenite grains is 5 μm or more. cannot be controlled. Therefore, S _GB /S _α is set to 0.70 or less. Preferably it is 0.50 or less.

In the metal structure of the steel sheet for hot stamping according to the present embodiment, the remaining structure is one or two or more types of pearlite, martensite, lower bainite, retained austenite, and tempered martensite. The area ratio of the remaining tissue should be greater than 50% and 90% or less from the relationship of S _α + S _GB .

Method for measuring metal structure of steel sheets for hot stamping

Samples are collected so that a cross-section of the plate thickness parallel to the rolling direction can be observed from an arbitrary position more than 50 mm away from the end surface of the hot stamping steel sheet (if the sample cannot be collected from this position, a position avoiding the end). Cut out. The size of the sample varies depending on the measuring device, but is set to a size that can be observed for about 10 mm in the rolling direction.

The cross section of the sample was polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using diamond powder with a particle size of 1 to 6 ㎛ dispersed in a diluent such as alcohol or pure water, followed by electrolytic polishing. Perform final polishing. Next, in order to be able to observe the position at a depth of 1/4 of the plate thickness from the surface, at an arbitrary position in the long side direction of the sample cross section, a length of 100 ㎛, a depth of 1/8 of the plate thickness from the surface to a depth of 1/8 of the plate thickness from the surface. In an area 3/8 of the thickness deep, the tissue is observed using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL) and an EBSD detector (DVC5 type detector, manufactured by TSL). The scanning electron microscope used is assumed to be equipped with two electron detectors. In a vacuum of 9.6×10 ^-5 Pa or less, an electron beam is irradiated to the sample at an acceleration voltage of 15 kV and an irradiation current level of 13, and a secondary electron image is photographed with a scanning electron microscope.

In the obtained photograph, the area where cementite is precipitated in a lamellar form within the particle is judged to be pearlite. The grains of the lath phase are judged to be lower bainite, martensite, and tempered martensite. Next, the same field of view is subjected to EBSD analysis at an analysis speed of 200 to 300 points/sec using an EBSD analysis device. Using the “Grain Average Misorientation” function included in the software “OIM Analysis (registered trademark)” attached to the EBSD analysis device, the area ratio S _α of ferrite and the area ratio S _GB of granular bainite are calculated. In this function, for crystal grains with a body-centered structure, it is possible to calculate the orientation difference between adjacent measurement points and then obtain the average value for all measurement points within the grain. Regarding the crystal orientation information obtained by EBSD analysis, the area surrounded by grain boundaries with an average crystal orientation difference of 5° or more is defined as a grain, and a map is drawn using the “Grain Average Misorientation” function. In the area excluding the areas determined as pearlite, lower bainite, martensite, and tempered martensite from the map, the area in which the average crystal orientation difference within the crystal grains is less than 0.4° is determined to be ferrite, and the average crystal orientation difference within the crystal grains is 0.4° or more. , the area below 3.0° is judged to be granular bainite. By calculating the area ratio of the area determined to be ferrite, the area ratio of ferrite is obtained. By calculating the area ratio of the region determined to be granular bainite, the area ratio of granular bainite is obtained.

The steel sheet for hot stamping according to the present embodiment may have a plating layer formed on the surface for the purpose of improving corrosion resistance after hot stamping. The plating layer may be either an electroplating layer or a hot-dip plating layer. The electroplating layer includes, for example, an electric zinc plating layer, an electric Zn-Ni alloy plating layer, etc. The hot-dip plating layer includes, for example, a hot-dip galvanizing layer, an alloyed hot-dip galvanizing layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, etc. . The adhesion amount of the plating layer is not particularly limited and may be a general adhesion amount.

In addition, the thickness of the hot stamping steel sheet according to the present embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body, etc.

Next, a hot stamped molded body according to the present embodiment, which is obtained by hot stamping the above-described hot stamping steel sheet, will be described. The hot stamp molded body according to the present embodiment has the same chemical composition as the steel sheet for hot stamping described above. The method for measuring the chemical composition may be the same as that used for hot stamping steel sheets. In addition, the hot stamp molded body according to the present embodiment has old austenite particles sized in the metal structure. That is, the hot stamped body according to the present embodiment has a metal structure in which the average particle size of the old austenite particles is 5 to 25 μm, and the standard deviation of the particle size of the old austenite particles is 0.1 to 2.0 μm.

In addition, in this embodiment, in a cross section perpendicular to the plate surface, at a position at a depth of 1/4 of the plate thickness from the surface (an area from a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface) Defines metal structure. The reason is that the metal structure at this position represents a typical metal structure of a hot stamp molded body. Hereinafter, the metal structure will be described.

“The average particle size of the old austenite particles is 5 to 25㎛”

“The standard deviation of the particle size of the old austenite particles is 0.1 to 2.0 μm.”

In the metal structure of the hot stamped body, the average particle size of the old austenite particles is set to 5 to 25 ㎛, and the standard deviation of the particle size of the old austenite particles is set to 0.1 to 2.0 ㎛, thereby improving the bendability of the hot stamped body. You can do it. If the average particle size of the old austenite particles or the standard deviation of the particle size of the old austenite particles is outside the above range, excellent bendability cannot be obtained in the hot stamped molded body.

The average particle diameter of the prior austenite particles is preferably 10 μm or more, and more preferably 15 μm or more. In addition, it is preferable that the average particle size of the prior austenite particles is 20 μm or less.

By setting the standard deviation of the particle size of the prior austenite particles to 2.0 μm or less, excellent bendability can be obtained in a hot stamped molded body. Therefore, the standard deviation of the particle size of the prior austenite particles is 2.0 μm or less. More preferably, it is 1.2 μm or less, even more preferably 1.1 μm or less, and even more preferably 0.4 μm or less.

In actual operation, it is difficult to make the standard deviation of the particle size of the old austenite particles less than 0.1 μm, so the practical lower limit is 0.1 μm or more.

If the area ratio of prior austenite particles with an average particle diameter of 0.5 to 3.0 μm is 60% or less, better bendability can be obtained in the hot stamped molded body. Therefore, the area ratio of old austenite particles having an average particle diameter of 0.5 to 3.0 μm may be 60% or less. More preferably, it is 50% or less, and even more preferably, it is 40% or less.

Method for measuring the average particle size and standard deviation of particle size of old austenite particles

Next, a method for measuring the average grain size of prior austenite particles will be explained. Samples are collected so that a cross-section of the sheet thickness parallel to the rolling direction can be observed from an arbitrary position 50 mm or more away from the end surface of the hot stamped body (if the sample cannot be collected from this position, a position avoiding the end). Cut it out. The size of the sample varies depending on the measuring device, but is set to a size that can be observed for about 10 mm in the rolling direction. The cross section of the sample was polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using diamond powder with a particle size of 1 to 6 ㎛ dispersed in a diluent such as alcohol or pure water, followed by electrolytic polishing. Perform final polishing using .

Next, in order to be able to observe a position at a depth of 1/4 of the sheet thickness from the surface, a depth of 1/8 of the sheet thickness from the surface to 3/8 of the sheet thickness from the surface is placed at an arbitrary position in the long side direction of the sample cross section. In the depth region, an area of 100 μm in length and 100 μm in the sheet thickness direction was measured using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL), 9.6 In a vacuum ^below Using the obtained crystal orientation information, the crystal orientation of the old austenite grains is calculated from the crystal orientation relationship between typical old austenite grains and grains with a body-centered structure after transformation, and using this, the average crystal grain size of the old austenite grains is calculated. Calculate

The method for calculating the crystal orientation of the old austenite grains is not particularly limited, but may be calculated by, for example, the following method. First, the crystal orientation of the old austenite grains is calculated by the method described in Non-Patent Document 1, and the crystal orientation of the old austenite in each coordinate of the region where EBSD was measured is specified. Next, using the “Inverse Pole Figure” function included in the software “OIM Analysis (registered trademark)” included with the EBSD analysis device, a crystal orientation map of the old austenite grains is created. For each old austenite particle included in the observation field, the average value of the shortest diameter and the longest diameter is calculated, and the average value is taken as the particle size of the old austenite particle. The above operation is performed on all old austenite particles, excluding old austenite particles whose entire crystal grains, such as the ends of the imaging field of view, are not included in the imaging field of view, and the grain size of all old austenite grains in the imaging field of view. Find . The average particle size of the old austenite particles in the imaging field of view is obtained by dividing the total particle size of the obtained old austenite particles by the total number of old austenite particles whose particle diameters were measured. This operation is performed for every imaged field of view, and the average particle size of the old austenite grains in the entire imaged field is calculated to obtain the average particle size of the old austenite grains.

By calculating the standard deviation from the particle size of the old austenite particles, the standard deviation of the particle size of the old austenite particles is obtained. At this time, in order to exclude the influence of locally generated fine grains or coarse grains, the standard deviation is calculated excluding the minimum and maximum values of the old austenite grain size.

By calculating the area of old austenite particles with an average particle size of 0.5 to 3.0 μm divided by the area of the entire measurement field of view, the area ratio of old austenite particles with an average particle size of 0.5 to 3.0 μm is obtained.

The metal structure of the hot stamped body is not particularly limited as long as the desired strength and bendability can be obtained after hot stamping, but for example, in area percentage, ferrite: 0 to 50%, bainite and martensite: 0 to 100%. , pearlite: 0 to 30%, and retained austenite: 0 to 5%. The metal structure of the hot stamp molded body can be measured by the following method.

Method for measuring metal structure of hot stamped body

A sample is cut from an arbitrary position 50 mm or more away from the end surface of the hot stamp molded body (if the sample cannot be taken from this position, a position avoiding the end surface) so that a cross section perpendicular to the plate surface can be observed. The cross section of this sample was polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using diamond powder with a particle size of 1 to 6 ㎛ dispersed in a diluent such as alcohol or pure water, and then polished to a mirror finish using a Nital product. Carry out etching. In order to be able to observe a position at a depth of 1/4 of the plate thickness from the surface, a length of 50 ㎛ at an arbitrary position in the long side direction of the sample cross section, a depth of 1/8 of the plate thickness from the surface to a depth of 1/8 of the plate thickness from the surface In an area of /8 depth, photographs of multiple views are taken using a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL). A grid at equal intervals is drawn on the photograph, and the tissue at the grid points is identified. By calculating the grid score corresponding to each organization and dividing it by the total grid score, the area ratio of each organization is obtained. The more total grid points there are, the more accurately the area ratio can be obtained. In this embodiment, the grid spacing is set to 2 μm x 2 μm, and the total number of grid points is set to 1500.

The area where cementite is precipitated in a lamellar form within the particle is judged to be pearlite. An area with low luminance and no underlying structure is judged to be ferrite. The area where the brightness is high and the underlying structure is not revealed by etching is judged to be martensite and retained austenite. The area that does not correspond to any of the above is judged to be bainite.

The area ratio of martensite is obtained by subtracting the area ratio of retained austenite determined by the EBSD analysis described later from the area ratio of martensite and retained austenite obtained from the above photograph.

The area ratio of retained austenite is measured by backscattered electron diffraction (EBSD). Analysis by EBSD is performed for an area from a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface, using samples collected at the same sampling location as the measurement using the above-mentioned photograph. . The sample was polished using #600 to #1500 silicon carbide paper, and then finished to a mirror finish using diamond powder with a particle size of 1 to 6 ㎛ dispersed in a diluent such as alcohol or pure water, and then the measurement cross section was polished to a mirror finish. It is assumed to have been finished by electrolytic polishing for the purpose of sufficiently removing distortion. In addition, in electrolytic polishing, in order to remove mechanical polishing deformation of the observation surface, a minimum of 20 μm may be polished, and a maximum of 50 μm may be polished. Considering the sagging of the ends, 30㎛ or less is preferable.

Measurements in EBSD are performed at an acceleration voltage of 15 to 25 kV, at intervals of at least 0.25 ㎛ or less, and crystal orientation information at each measurement point in a range of 150 ㎛ or more in the sheet thickness direction and 250 ㎛ or more in the rolling direction. get Among the obtained crystal structures, those with a crystal structure of fcc are determined to be retained austenite using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. By determining the ratio of measurement points determined to be retained austenite, the area ratio of retained austenite is obtained. Here, since it is preferable to have more measurement points, it is better for the measurement interval to be narrow and the measurement range to be wide. However, when the measurement interval is less than 0.01 μm, adjacent points interfere with the diffusion width of the electron beam. Therefore, the measurement interval is set to 0.01 μm or more. Additionally, the maximum measurement range should be 200 ㎛ in the sheet thickness direction and 400 ㎛ in the sheet width direction. In addition, for the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL) and an EBSD detector (DVC5 type detector, manufactured by TSL) is used. At this time, the vacuum level within the device is 9.6×10 ^-5 Pa or less, the irradiation current level is 13, and the electron beam irradiation level is 62.

The hot stamped molded body according to the present embodiment may have a plating layer formed on the surface for the purpose of improving corrosion resistance after hot stamping. The plating layer may be either an electroplating layer or a hot-dip plating layer. The electroplating layer includes, for example, an electric zinc plating layer, an electric Zn-Ni alloy plating layer, etc. The hot-dip plating layer includes, for example, a hot-dip galvanizing layer, an alloyed hot-dip galvanizing layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, etc. . The adhesion amount of the plating layer is not particularly limited and may be a general adhesion amount.

The plate thickness of the hot stamp molded body according to the present embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body, etc.

The hot stamp molded body according to this embodiment has a tensile (maximum) strength of 2200 MPa or more. Preferably, it is 2400 MPa or more, and more preferably, it is 2550 MPa or more. The tensile strength is obtained by producing a No. 5 test piece described in JIS Z 2241:2011 from the flattest possible position of the hot stamp molded body and following the test method described in JIS Z 2241:2011.

In addition, the hot stamp molded body according to the present embodiment preferably has a maximum bending angle of 20° or more obtained by a bending test based on the VDA standard (VDA238-100) specified by the German Automobile Manufacturers Association. More preferably, it is 30° or more or 40° or more. The conditions for the bending test are as follows.

Test piece dimensions: 60 mm (rolling direction) × 30 mm (direction parallel to the sheet width direction)

Test piece plate thickness: 1.6 mm

Bending ridge: Direction parallel to the direction of the plate width.

Test method: roll supported, punch press fit

Roll diameter: ø30mm

Punch shape: tip R=0.4mm

Distance between rolls: 2.0 × plate thickness (mm) + 0.5 mm

Indentation speed: 20㎜/min

Tester: SHIMADZU AUTOGRAPH 20kN

Next, a method for manufacturing a steel sheet for hot stamping according to this embodiment will be described.

In the method for manufacturing a hot stamping steel sheet according to the present embodiment, in order to obtain a hot stamping steel sheet having the above-described metal structure, it is preferable that the final reduction ratio of the finish rolling in hot rolling is set to 40 to 80%. Normally, the final reduction ratio of finish rolling is less than 10%, but in this embodiment, it is preferable to set the final reduction ratio higher than the normal final reduction ratio.

The steel piece (steel material) used for hot rolling may be a steel piece manufactured by a conventional method, for example, a steel piece manufactured by a general method such as a continuous casting slab or a thin slab caster. In addition, in the casting process, the steel piece after solidification may be rolled at a reduction ratio of 30 to 70% in a temperature range where the center temperature of the slab is 1200°C or higher and the solidus temperature or lower. As a result, segregation of Mn is alleviated, and bendability can be improved in the hot stamp molded body. In addition, the solidus temperature can be obtained from the following formula (1).

Solidus temperature (°C) = 1536-(415.5 … (One)

In addition, in the above formula (1), %C, %Si, %Mn, %P, %S, %Ni, %Cr and %Al mean the content (% by mass) of each element.

In hot rolling, rough rolling and finish rolling are performed. In finish rolling, the slab after rough rolling is rolled by a plurality of finish rolling mills. In this embodiment, it is preferable to perform finish rolling so that the reduction ratio (final reduction ratio) in the final pass of finish rolling is 40% or more. The final reduction ratio is {(t ₀ -t ₁ )/t ₀ }×100(%, when the plate thickness before the final pass of finish rolling is t ₀ and the plate thickness after the final pass of finish rolling is t ₁ ) can be expressed as

By setting the final reduction ratio of finish rolling to 40 to 80%, the prior austenite particles are refined and the origin of ferrite and granular bainite increases. As a result, in the metal structure of the steel sheet for hot stamping, S _α +S _GB and S _GB /S _α can be within the desired range. If the final reduction ratio of the finish rolling is less than 40%, S _α + S _GB and S _GB /S _α cannot be within the desired range in the metal structure of the steel sheet for hot stamping. Therefore, it is preferable that the final reduction ratio of finish rolling is 40% or more. The final reduction ratio of finish rolling is preferably 50% or more. On the other hand, if the final reduction ratio of finish rolling exceeds 80%, S _GB /S _α cannot be controlled to 0.70 or less. Therefore, it is preferable that the final reduction ratio of finish rolling is 80% or less. More preferably, it is less than 70%.

In addition, the heating temperature and holding time of the steel piece before hot rolling are not particularly limited, but are preferably held in a temperature range of 1200°C or higher for 20 minutes or more.

After finish rolling, it is preferable to coil in a temperature range of 400 to 750°C. If the coiling temperature exceeds 750°C, ferrite transformation is too accelerated, and S _α +S _GB becomes 50% or more and S _GB /S _α becomes less than 0.30. The coiling temperature is preferably 700°C or lower, and more preferably 660°C or lower.

Additionally, the coiling temperature is preferably 400°C or higher. If the coiling temperature is less than 400°C, the formation of granular bainite is suppressed and S _GB /S _α becomes less than 0.30. The coiling temperature is preferably 450°C or higher, and more preferably 530°C or higher.

In addition, after finish rolling (after completion of hot rolling), it is preferable to cool after 2.5 seconds or more. The cooling referred to here does not include air cooling and refers to cooling with an average cooling rate of 50 to 200°C/s. If the time from finish rolling to the start of cooling is less than 2.5 seconds, the desired amount of S _α + S _GB may not be obtained.

After winding, cold rolling may be performed as needed. Additionally, the above-mentioned plating may be formed after finish rolling or cold rolling. Additionally, pickling may be performed between hot rolling and cold rolling. In cold rolling, a normal cumulative reduction ratio may be set, for example, 30 to 90%. Additionally, temper rolling may be performed under normal conditions. Additionally, for the purpose of softening the hot-rolled steel sheet, annealing of the hot-rolled steel sheet may be performed by heating the hot-rolled steel sheet to a temperature range of 730°C or lower.

By the above method, the steel sheet for hot stamping according to this embodiment can be manufactured. Next, a method for manufacturing a hot stamp molded body according to the present embodiment, which can be manufactured using the above-described hot stamp steel sheet, will be described. The manufacturing method of the hot stamp molded body according to the present embodiment is not particularly limited, but the following manufacturing method may be used, for example.

First, the steel sheet for hot stamping described above is heated to a temperature range of 800°C or higher. If the heating temperature is less than 800°C, coarse carbides may remain during heating and the bendability of the hot stamped body may decrease. The heating temperature is preferably 820°C or higher, and more preferably 860°C or higher.

The upper limit of the heating temperature is not particularly limited, but if the heating temperature is too high, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot stamped body is reduced. Therefore, the heating temperature is preferably 1000°C or lower, more preferably 960°C or lower, and even more preferably 930°C or lower.

In addition, the holding time at the above heating temperature is preferably 1.0 to 10.0 minutes. If the holding time is less than 1.0 minutes, coarse carbides may remain and the bendability of the hot stamped body may decrease. On the other hand, if the holding time exceeds 10.0 minutes, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot stamped body may decrease.

In addition, it is preferable that the average heating rate up to the above heating temperature is 1.0°C/s or more. If the average heating rate is less than 1.0°C/s, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot stamped body decreases. The upper limit is not specifically determined, but since it is difficult to exceed 1000°C/s in actual operation, 1000°C/s or less is the practical upper limit.

After the above-described heating and holding, hot stamping is performed. After hot stamping, it is preferable to cool, for example, to a temperature range of 300°C or lower at an average cooling rate of 10°C/s or more. If the average cooling rate is less than 10°C/s, strength may be insufficient. The upper limit is not specifically determined, but since it is difficult to exceed 1000°C/s in the actual fishing industry, 1000°C/s or less is the practical upper limit.

Additionally, in heating during hot stamping, it is not desirable to perform preliminary heating, that is, two-stage heating. The carbon segregation area at the grain boundary created in the hot stamping steel sheet stage is eliminated, making it impossible to uniformly disperse and generate old austenite particles, and as a result, the standard deviation of old austenite particles cannot be controlled within the desired range. Because there is none.

The hot stamp molded body according to the present embodiment can be obtained by the preferred manufacturing method described above. Additionally, treatment may be performed by tempering at 150 to 600°C after hot stamping. Additionally, a part of the hot stamp molded body may be tempered by laser irradiation or the like to partially provide a softened region. Weldability improves in the softened area. For example, if spot welding is performed after softening the end of a hot stamp molded body, the difference in strength between the softened end and the spot welded area in the end can be reduced, thereby suppressing destruction from the interface between the two. Additionally, for example, when applying a hot stamp molded body to a high-strength member of an automobile, the failure and deformation mode of the high-strength member during a collision can be controlled by providing a softening area in a part of the high-strength member.

Example

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are examples of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this example of conditions. no. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

A steel piece manufactured by casting molten steel with the chemical composition shown in Tables 1A to 1D is heated, held in a temperature range of 1200°C or higher and below 1350°C for 20 minutes or more, and then hot rolled under the conditions shown in Tables 2A to 2F. , cooling and winding were performed, and as necessary, cold rolling, hot-rolled sheet annealing, pickling, and plating were performed. As a result, steel sheets for hot stamping shown in Tables 2A to 2F were obtained. In addition, the average cooling rate for cooling after finish rolling until coiling was 50 to 200°C/s. In addition, after finish rolling, cooling was performed at the above-mentioned average cooling rate after 2.5 seconds or more had elapsed. However, steel plate No. 172 marked with “*” was cooled 2.0 seconds after finish rolling.

In addition, for steel plate No. 107, in the casting process, the steel piece after solidification was rolled at a reduction ratio of 30 to 70% in a temperature range where the center temperature of the slab is below the solidus temperature.

For steel plate No. 108, the heating temperature before hot rolling was 1350°C.

Steel plate No. 125 was subjected to hot-rolled sheet annealing by heating and maintaining the temperature range at 730°C or lower.

Steel plate No. 126 was not cold rolled.

Steel plate No. 127 had an electro-galvanized layer formed on its surface.

Steel plate No. 128 had an electric Zn-Ni alloy plating layer formed on its surface.

Steel plate No. 129 had a hot-dip galvanized layer formed on its surface.

Steel plate No. 130 had an alloyed hot-dip galvanized layer formed on its surface.

Steel plate No. 131 had a molten aluminum plating layer formed on its surface.

Steel plate No. 132 had a molten Zn-Al alloy plating layer formed on its surface.

Steel plate No. 133 had a molten Zn-Al-Mg alloy plating layer formed on its surface.

Steel plate No. 134 had a molten Zn-Al-Mg-Si alloy plating layer formed on its surface.

The obtained steel sheet for hot stamping was hot stamped under the conditions shown in Tables 3A to 3F, and hot stamped molded bodies shown in Tables 3A to 3F were obtained.

Production No. 161 was subjected to tempering treatment at 150 to 600°C after hot stamping.

In production No. 162, a part of the hot stamp molded body was tempered by laser irradiation to form a partially softened area.

Production No. 163 was heated to the heating temperature shown in Table 3F, then cooled to a temperature range of 250°C or lower, then heated to 900°C, and then cooled at the average cooling rate in Table 3D by hot stamping.

In addition, in the present invention examples of Tables 2A to 2F, the residual structure is one or two or more types of pearlite, martensite, lower bainite, retained austenite, and tempered martensite, and the area ratio of the sum of these is , it was more than 50% and less than 90%. In addition, in the present invention examples of Tables 3A to 3F, the metal structure, in area percentage, is ferrite: 0 to 50%, bainite and martensite: 0 to 100%, pearlite: 0 to 30%, and residual Austenite: It consisted of 0 to 5%.

In addition, the method for measuring the metal structure of the steel sheet for hot stamping and the method for measuring the metal structure and mechanical properties of the hot stamping molded body were as described above. If the tensile strength of the hot stamp molded body was 2200 MPa or more, it was judged to have high strength and was judged as passing. If the tensile strength was less than 2200 MPa, it was judged to be failed as it did not have high strength.

In addition, when the maximum bending angle was 20° or more, it was judged as having excellent bendability and was judged as passing, and when the maximum bending angle was less than 20°, it was judged as failing because it did not have excellent bending properties.

[Table 1A]

[Table 1B]

[Table 1C]

[Table 1D]

[Table 2A]

[Table 2B]

[Table 2C]

[Table 2D]

[Table 2E]

[Table 2F]

[Table 3A]

[Table 3B]

[Table 3C]

[Table 3D]

[Table 3E]

[Table 3F]

Looking at Tables 3A to 3F, it can be seen that the hot stamp molded body according to the present invention example has high strength and excellent bendability. On the other hand, it can be seen that one of the properties of the hot stamp molded body according to the comparative example was deteriorated.

According to the above aspect of the present invention, a hot stamp molded body having high strength and excellent bendability, and a steel sheet for hot stamping from which this hot stamp molded body can be manufactured can be provided.

Claims (5)

Chemical composition, in mass%,
C: greater than 0.40%, less than or equal to 0.70%,
Si: 0.010 to 1.30%,
Mn: more than 0.60%, less than 3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0130% or less,
O: 0.0200% or less,
Al: 0.0010 to 0.500%,
Cr: 0.010 to 0.80%,
Nb: 0 to 0.100%,
Ti: 0 to 0.100%,
B: 0 to 0.0100%,
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0% or more, less than 3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0 to 1.000%,
Ca: 0 to 0.010%,
Mg: 0 to 1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0 to 1.000%, and
As: 0 to 0.100%
Contains, the balance consists of Fe and impurities,
S _α + S GB _, which is the sum of the area ratio S _α of ferrite and the area ratio S _GB of granular bainite, is 10% or more and less than 50%,
A steel sheet for hot stamping, characterized in that it has a metal structure in which S _GB /S _α , which is a ratio of the area ratio S _GB of the granular bainite and the area ratio S _α of the ferrite, is 0.30 to 0.70. According to paragraph 1,
The chemical composition is expressed in mass%,
Nb: 0.001 to 0.100%,
Ti: 0.010 to 0.100%,
B: 0.0015 to 0.0100%,
Mo: 0.05 to 1.00%,
Co: 0.05 to 2.00%,
Ni: 0.01% or more, less than 3.00%,
Cu: 0.01 to 1.00%,
V: 0.01 to 1.00%,
W: 0.001 to 1.000%,
Ca: 0.001 to 0.010%,
Mg: 0.001 to 1.000%,
REM: 0.001 to 1.000%,
Sb: 0.005 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001 to 1.000%, and
As: 0.001 to 0.100%
A steel sheet for hot stamping, characterized in that it contains one or two or more types selected from the group consisting of. Chemical composition, in mass%,
C: greater than 0.40%, less than or equal to 0.70%,
Si: 0.010 to 1.30%,
Mn: more than 0.60%, less than 3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0130% or less,
O: 0.0200% or less,
Al: 0.0010 to 0.500%,
Cr: 0.010 to 0.80%,
Nb: 0 to 0.100%,
Ti: 0 to 0.100%,
B: 0 to 0.0100%,
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0% or more, less than 3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0 to 1.000%,
Ca: 0 to 0.010%,
Mg: 0 to 1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0 to 1.000%, and
As: 0 to 0.100%
Contains, the balance consists of Fe and impurities,
It has a metal structure in which the average particle size of the old austenite particles is 5 to 25 ㎛, and the standard deviation of the particle size of the old austenite particles is 0.1 to 2.0 ㎛,
A hot stamp molded body characterized by a tensile strength of 2200 MPa or more. According to paragraph 3,
The chemical composition is expressed in mass%,
Nb: 0.001 to 0.100%,
Ti: 0.010 to 0.100%,
B: 0.0015 to 0.0100%,
Mo: 0.05 to 1.00%,
Co: 0.05 to 2.00%,
Ni: 0.01% or more, less than 3.00%,
Cu: 0.01 to 1.00%,
V: 0.01 to 1.00%,
W: 0.001 to 1.000%,
Ca: 0.001 to 0.010%,
Mg: 0.001 to 1.000%,
REM: 0.001 to 1.000%,
Sb: 0.005 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001 to 1.000%, and
As: 0.001 to 0.100%
A hot stamp molded body characterized in that it contains one or two or more types selected from the group consisting of. According to clause 3 or 4,
A hot stamp molded body, characterized in that the area ratio of the old austenite particles having an average particle diameter of 0.5 to 3.0 ㎛ is 60% or less.