Hot-Stamp Formed Body

The present invention relates to a hot-stamp formed body.

Priority is claimed on Japanese Patent Application No. 2022-067026, filed Apr. 14, 2022, the content of which is incorporated herein by reference.

In recent years, there has been a demand for a reduction in the weight of a vehicle body in terms of environmental protection and resource saving, and a high-strength steel sheet has been applied to vehicle members. When a high-strength steel sheet is applied, the desired strength can be imparted to vehicle bodies while reducing the thickness of the steel sheet and reducing the weight of the vehicle bodies. Vehicle members are manufactured by press forming steel sheets, but not only a forming load is increased but also the formability deteriorates as the strength of a steel sheet is increased. Furthermore, when press forming a high-strength steel sheet, since a shape of a member changes significantly due to spring-back when the member is taken out of a mold, it is difficult to ensure the dimensional accuracy of the member. And thus, it is not easy to manufacture vehicle members with the high-strength by press forming.

In order to solve this issue, until now, for example, as disclosed in Patent Document 1, a technique has been proposed in which press forming a heated steel sheet using a press mold with low temperature. This technique is called hot stamping or hot pressing, and since the steel sheet is press formed in a soft state by being heating to a high temperature, it is possible to manufacture members having complex shapes with high dimensional accuracy. In addition, since the steel sheet is rapidly cooled by contact with the mold, it is possible to significantly increase the strength by quenching at the same time as press forming. Patent Document 1 discloses that a member having a tensile strength of 1400 MPa or more can be obtained by hot stamping a steel sheet having a tensile strength of 500 MPa to 600 MPa.

The strength of a hot-stamp formed member can be further increased by increasing a C content of a steel sheet. However, when a C content of a steel sheet is increased, the deformability of a member decreases as the strength of the member increases, and when the member deforms during a collision, a cracking becomes easy to occur at an initial stage of deformation. And thus, it is not easy to manufacture a high-strength hot-stamp formed member with excellent collision resistance property, and especially when the tensile strength of the member exceeds 1900 MPa, it becomes difficult to achieve both strength and collision resistance property.

As a technology for manufacturing a hot stamping member with excellent collision resistance property, Patent Document 2 discloses a high-strength pressed component having a tensile strength of 1300 MPa or more and high impact absorbability, and a method for manufacturing the same. Furthermore, Patent Document 3 discloses a hot-stamping member for a vehicle, which has a tensile strength of 1100 MPa or more and has improved bendability from the viewpoint of absorbing impact energy, and a method for manufacturing the same. In the methods disclosed in Patent Document 2 and Patent Document 3, a steel sheet for hot stamping having a decarburized layer on a surface layer is subjected to hot stamping under predetermined conditions, a soft layer is formed on the surface layer of the hot stamping member, and thereby improving the collision resistance property of the component.

However, according to studies conducted by the present inventors, it has been found that when hot stamping is performed using steel sheets whose surface layer is decarburized, C flows from the inside of the steel sheets into the surface layer during a heating process of hot stamping, and the C concentration in the decarburized layer increases, the surface layer of the member does not soften and the collision resistance property may not be sufficiently improved.

The present invention has been made in view of the circumstances described above, and an object of the present invention is to provide a hot-stamp formed body having high strength with tensile strength of 1900 MPa or more and excellent collision resistance property.

The gist of the present invention is as follows.

(1) A hot-stamp formed body according to an aspect of the present invention is a hot-stamp formed body comprising a steel sheet, an entirety or a part of the steel sheet comprises, as a chemical composition, by mass %:

(2) In the hot-stamp formed body according to (1), the chemical composition may comprise, by mass %, one or two or more selected from the group consisting of:

According to the above-described aspect of the present invention, it is possible to provide a hot-stamp formed body having high strength of tensile strength of 1900 MPa or more and excellent collision resistance property. The hot-stamp formed body according to the above-described aspect has excellent collision resistance property and does not crack at an initial stage of deformation, and thereby suitably applying to vehicle members of pillars, bumpers, and so on.

The present inventors has studied a method for suppressing the occurrence of a cracking at an initial stage of deformation during a collision in a hot-stamp formed body having a tensile strength of 1900 MPa or more, and as a result, the following findings were obtained.

(A) By softening a surface layer of a hot-stamp formed body, the collision resistance property of the hot-stamp formed body is improved.

(B) In order to soften a surface layer of a hot-stamp formed body, it is effective to perform hot stamping using a steel sheet for hot stamping having a decarburized layer in a surface layer. However, in the process of heating a steel sheet for hot stamping, C diffuses from the inside of the steel sheet to the surface layer, and a phenomenon (referred to as recarburization) in which the C concentration in the surface layer increases occurs. Therefore, a surface layer of a hot-stamp formed body does not sufficiently soften, and the collision resistance property of the hot-stamp formed body may not be sufficiently improved.

(C) In the hot-stamp formed body, (a) by decreasing an average B concentration in a region from a depth of 5.0 μm from a surface of a steel sheet constituting the hot-stamp formed body to a depth of 25.0 μm from the surface, (b) by increasing an average B concentration in a region from a depth of 0.5 μm from the surface to a depth of 4.0 μm from the surface, and (c) by increasing an average O concentration in the region from the depth of 0.5 μm from the surface to the depth of 4.0 μm from the surface, it is possible to improve the collision resistance property of the hot-stamp formed body even if recarburization occurs during heating process of hot stamping.

(D) Although the reason is not clear, it is presumed that (a) in the region from the depth of 5.0 μm from the surface of the steel sheet constituting the hot-stamp formed body to the depth of 25.0 μm from the surface, the hardenability of the steel sheet decreases due to a decrease of the average B concentration, (b) in the region from the depth of 0.5 μm from the surface to the depth of 4.0 μm from the surface, B and O combine and the hardenability of the steel sheet decreases, and thereby softening the surface layer even if the C concentration in the surface layer of the hot-stamp formed body increases due to the recarburization.

Based on the findings of (A) to (D), the present inventors found that by adjusting the B concentration and O concentration in the surface layer (including a surface layer region and an outermost layer region described below) of the hot-stamp formed body to a specific range, a hot-stamp formed body having a tensile strength of 1900 MPa or more and excellent collision resistance property can be obtained.

Hereinafter, the hot-stamp formed body according to the present embodiment will be described in detail. First, the reason why the chemical composition of the steel sheet constituting the hot-stamp formed body according to the present embodiment is limited will be described.

An entirety or a part of the steel sheet constituting the hot-stamp formed body according to the present embodiment has the following chemical composition. When the hot-stamp formed body consists of only the steel sheet, an entirety or a part of the hot-stamp formed body has the chemical composition shown below.

Note that a limited numerical range described using “to” described below includes a lower limit and an upper limit. Numerical values represented using “less than” or “more than” are not included in a numerical range. All % related to the chemical composition mean mass %.

When the hot-stamp formed body has a part having a tensile strength of 1900 MPa or more and a part having a tensile strength of less than 1900 MPa, at least the part having a tensile strength of 1900 MPa or more may have the following chemical composition.

an entirety or a part of the steel sheet constituting the hot-stamp formed body according to the present embodiment comprises, as a chemical composition, by mass %, C; more than 0.32% and 0.70% or less, Si: less than 2.00%, Mn: 0.01% to 3.00%, P; 0.200% or less, S: 0.0200% or less, sol. Al: 0.001% to 1.000%, N: 0.0200% or less, O: 0.0005% to 0.0200%, B: 0.0005% to 0.0200%, and a remainder of Fe and impurities.

Each element will be described below.

C: More than 0.32% and 0.70% or Less

C is an element that improves the strength of the steel sheet after hot stamping (the steel sheet constituting the hot-stamp formed body). When the C content is 0.32% or less, a tensile strength of the steel sheet after hot stamping becomes less than 1900 MPa, and the strength of the hot-stamp formed body becomes insufficient. In addition, collision resistance property of the hot-stamp formed body deteriorates. Therefore, the C content is set to more than 0.32%. The C content is preferably more than 0.34%, more than 0.38%, more than 0.42% or more than 0.45%.

On the other hand, when the C content is more than 0.70%, the strength of the hot-stamp formed body becomes too high, and excellent collision resistance property cannot be obtained. Therefore, the C content is set to 0.70% or less. The C content is preferably 0.65% or less, 0.60% or less, 0.55% or less or 0.50% or less.

Si: Less than 2.00%

Si is an element that may be comprised as an impurity in steel, and makes steel brittle. When the Si content exceeds 2.00%, the adverse effects become particularly significant. Therefore, the Si content is set to less than 2.00%. The Si content is preferably less than 1.00%, less than 0.75%, less than 0.50% or less than 0.20%.

The lower limit of the Si content is not particularly limited, but may be 0%. Since excessively lowering the Si content causes an increase in steel manufacturing costs, the Si content is preferably set to 0.001% or more. In addition, since Si has the effect of improving the hardenability of steel, Si may be actively comprised. From the viewpoint of improving the hardenability, the Si content is preferably 0.05% or more, 0.10% or more or 0.15% or more.

Mn is an element that combines with S, which is an impurity, to form MnS and has the effect of suppressing the harmful effects of S. When the Mn content is less than 0.01%, the above effects cannot be obtained. Therefore, the Mn content is set to 0.01% or more. In addition, Mn is an element that improves the hardenability of steel and forms a microstructure mainly composed of martensite inside the steel sheet after hot stamping, and an effective element for ensuring the strength of the hot-stamp formed body. From the viewpoint of ensuring the strength, the Mn content is preferably 0.50% or more, 0.75% or more, 1.00% or more or 1.25% or more.

On the other hand, when the Mn content is more than 3.00%, excellent collision resistance property in the hot-stamp formed body cannot be obtained. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.00% or less or 1.50% or less.

P is an element that may be comprised as an impurity in steel, and makes steel brittle. When the P content exceeds 0.200%, the adverse effects become particularly significant, and the weldability deteriorates significantly. Therefore, the P content is set to 0.200% or less. The P content is preferably less than 0.100%, less than 0.050% or less than 0.020%.

The P content may be 0%, but the dephosphorization cost increases significantly when the P content is reduced to less than 0.001%, which is not preferable economically. Therefore, the P content may be set to 0.001% or more or 0.005% or more.

S is an element that may be comprised as an impurity in steel, and makes steel brittle. When the S content exceeds 0.0200%, the adverse effects become particularly significant. Therefore, the S content is set to 0.0200% or less. The S content is preferably less than 0.0050%, less than 0.0020% or less than 0.0010%.

The S content may be 0%, but the desulfurization cost increases significantly when the S content is reduced to less than 0.0001%, which is not preferable economically. Therefore, the S content may be set to 0.0001% or more or 0.0002% or more.

Sol. Al: 0.001% to 1.000%

Al is an element having an effect of deoxidizing molten steel. When 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 sol. Al content is preferably 0.005% or more, 0.010% or more or 0.020% or more.

On the other hand, when the sol. Al content is too high, a transformation point increases, and it becomes difficult to heat the steel sheet to a temperature of higher than an Acpoint in heating step of hot stamping. In addition, the strength and the collision resistance property of the hot-stamp formed body deteriorate. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is preferably less than 0.500%, less than 0.100%, less than 0.060% or less than 0.040%.

N is an element that may be comprised as an impurity in steel, and forms nitrides during continuous casting of steel. Since the nitrides deteriorate ductility of the steel sheet after hot stamping, it is preferable that the N content is lower. When the N content exceeds 0.0200%, the adverse effects become particularly significant. Therefore, the N content is set to 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080% or less than 0.0050%.

The N content may be 0%, but the denitrification cost increases significantly when the N content is reduced excessively, which is not preferable economically. Therefore, the N content may be set to 0.0005% or more, or 0.0010% or more or 0.0020% or more.

O is an element that forms B oxide by combining with B and reduces the hardenability of steel, and is an effective element for softening the surface layer of the steel sheet after hot stamping. When the O content is less than 0.0005%, the surface layer of the steel sheet after hot stamping does not soften, and the collision resistance property in the hot-stamp formed body deteriorates. Therefore, the O content is set to 0.0005% or more. The O content is preferably 0.0010% or more, 0.0015% or more or 0.0020% or more.

On the other hand, when the O content is more than 0.0200%, a large amount of coarse oxide inclusions is formed in steel. And thus, the collision resistance property of the hot-stamp formed body deteriorates. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less or 0.0040% or less.

B is an element that improves the hardenability of steel and forms a microstructure mainly composed of martensite inside the steel sheet after hot stamping, and an effective element for ensuring the strength of the hot-stamp formed body. When the B content is less than 0.0005%, the desired strength in the hot-stamp formed body cannot be obtained. In addition, the collision resistance property of the hot-stamp formed body deteriorates. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more, 0.0015% or more or 0.0020% or more.

On the other hand, when the B content is more than 0.0200%, carborides are formed in the hot-stamp formed body, and the effect of hardenability improvement of B is impaired. Therefore, the B content is set to 0.0200% or less. The B content is preferably less than 0.0050%, less than 0.0040% or less than 0.0030% or less,

The remainder of the chemical composition of the steel sheet constituting the hot-stamp formed body according to the present embodiment may be Fe and impurities. Elements, which are unavoidably mixed from a steel raw material or scrap and/or during the manufacture of steel and are allowed in a range where the properties of the hot-stamp formed body according to the present embodiment do not deteriorate, are exemplary examples of the impurities.

The steel sheet constituting the hot-stamp formed body according to the present embodiment may comprise the following elements as optional elements instead of a part of Fe. The content of the following optional elements obtained in a case where the following optional elements are not contained is 0%.

Cr is an element that increases the strength of the hot-stamp formed body by increasing the hardenability of steel. In order to reliably obtain the effect, the Cr content is preferably set to 0.01% or more. The Cr content is more preferably 0.05% or more or 0.10% or more.

On the other hand, when the Cr content is more than 2.00%, the collision resistance property of the hot-stamp formed body deteriorates. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably less than 0.50%, less than 0.40% or less than 0.30%,