WO2023171492A1

WO2023171492A1 - Hot-stamp-formed article

Info

Publication number: WO2023171492A1
Application number: PCT/JP2023/007562
Authority: WO
Inventors: 皓大村澤; 真吾藤中; 由梨戸田
Original assignee: 日本製鉄株式会社
Priority date: 2022-03-11
Filing date: 2023-03-01
Publication date: 2023-09-14

Abstract

This hot-stamp-formed article has a specific chemical composition, with the metal structure being, by area%: at least 60% in total of fresh martensite and tempered martensite; 2 to 30% retained austenite; 38% or less of bainite; and 2% or less in total of ferrite and pearlite. The metal structure is configured so so that ρS/ρO is 0.95 to 1.01; fS/fO is 0.80 to 1.20; and βS/βO is 0.990 to 1.010.

Description

hot stamp molded body

The present invention relates to a hot stamp molded article.
This application claims priority based on Japanese Patent Application No. 2022-037893 filed in Japan on March 11, 2022, the contents of which are incorporated herein.

In recent years, there has been a demand for lighter automobile bodies from the viewpoint of environmental protection and resource conservation, and high-strength steel plates are being applied to automobile parts.

For example, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet that has high strength and excellent delayed fracture resistance, and a method for manufacturing the same.

Patent Document 2 states that by controlling the internal oxidation depth and the amount of retained austenite after primary annealing, yield strength and ductility are improved, and stable production and provision can be achieved without dent defects occurring during production. A high-strength cold-rolled steel sheet, a plated steel sheet, and a method for manufacturing these are disclosed.

Automotive parts are manufactured by press forming, but as the strength of steel plates increases, not only does the forming load increase, but also formability decreases. Therefore, in high-strength steel sheets, formability into members with complex shapes becomes an issue.

In order to solve the above-mentioned problems, the application of hot stamping technology is progressing, in which press forming is performed after heating the steel plate to a high temperature in the austenite region where the steel plate becomes soft. Hot stamping is attracting attention as a technology that achieves both moldability into automobile parts and strength of automobile parts by performing quenching treatment in a mold at the same time as press working.

For example, Patent Document 3 discloses a collision reinforcement material such as a bracket-integrated door impact beam, which can be given desired strength by press working, and a method for manufacturing the same.

International Publication No. 2020/136990 Japan Special Table No. 2019-512608 Japanese Patent Application Publication No. 2002-102980

However, Patent Documents 1 to 3 do not consider the ductility after hot stamping.

The present invention has been made in view of the above problems. An object of the present invention is to provide a hot-stamped molded article having high strength and excellent ductility.

The gist of the invention is as follows.
(1) The hot stamp molded article according to one embodiment of the present invention has a chemical composition in mass %,
C: 0.08-0.70%,
Si: 0.100-3.000%,
Mn: 0.100-3.000%,
P: 0.1000% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.1000% or less,
Al: 3.0000% or less,
B: 0.0005-0.0200%,
Nb: 0 to 0.100%,
Ti: 0-0.200%,
Cr: 0-1.00%,
Mo: 0-1.00%,
Co: 0-5.00%,
Ni: 0-3.00%,
Cu: 0-3.00%,
V: 0 to 3.00%,
W: 0-3.00%,
Ca: 0-1.0000%,
Mg: 0 to 1.0000%,
REM: 0-1.0000%,
Sb: 0 to 1.00%,
Zr: 0 to 1.00%,
Contains Sn: 0 to 1.00% and As: 0 to 1.0000%,
The remainder consists of Fe and impurities,
In metallographic structure,
In area%,
Fresh martensite and tempered martensite: 60% or more in total,
Retained austenite: 2-30%,
Bainite: 38% or less, and
Ferrite and pearlite: 2% or less in total,
When the number density of the retained austenite is ρ ₀ , and the number density of the retained austenite after sub-zero treatment is ρ _S , ρ _S /ρ ₀ is 0.95 to 1.01,
The total area ratio of the fresh martensite and the tempered martensite having a grain size of 5 to 10 μm and a GAM value of 2° or more is _f0 , and the grain size is 5 to 10 μm, and When the sum of the area ratios of the fresh martensite and the tempered martensite after the sub-zero treatment with a GAM value of 2° or more is f _S , f _S /f ₀ is 0.80 to 1.20. ,
The half-width of the fresh martensite and the tempered martensite obtained by X-ray diffraction is β ₀ , and the half-width of the fresh martensite and the tempered martensite after the sub-zero treatment obtained by X-ray diffraction is When β _S is set, β _S /β ₀ is 0.990 to 1.010.
(2) The hot-stamped molded article according to (1) above has the chemical composition in mass %,
Nb: 0.001 to 0.100%,
Ti: 0.001 to 0.200%,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
Co: 0.01-5.00%,
Ni: 0.01 to 3.00%,
Cu: 0.01-3.00%,
V: 0.01 to 3.00%,
W: 0.01-3.00%,
Ca: 0.0001-1.0000%,
Mg: 0.0001 to 1.0000%,
REM: 0.0001-1.0000%,
Sb: 0.01 to 1.00%,
Zr: 0.01-1.00%,
Sn: 0.01 to 1.00%, and As: 0.0001 to 1.0000%
It may contain one or more selected from the group consisting of:

According to the above aspect of the present invention, it is possible to provide a hot-stamped molded article having high strength and excellent ductility.

The present inventors have studied methods for improving ductility in high-strength hot-stamped molded bodies, and have obtained the following findings.

By including a predetermined amount of retained austenite in the metal structure, the ductility of the hot-stamped compact is improved due to deformation-induced transformation. Furthermore, the greater the strain at which deformation-induced transformation of retained austenite occurs, the greater the effect of improving ductility can be obtained. In deformation-induced transformation, when retained austenite, which is less stable among retained austenites, begins to transform, the transformation of surrounding retained austenite is promoted one after another using this transformation as a core. Therefore, by reducing the amount of residual austenite, which has low stability in the metallographic structure of the hot-stamped compact, it becomes difficult for deformation-induced transformation to occur when a small strain is introduced, and the strain at which transformation of retained austenite starts becomes larger. This improves the ductility of the hot stamped product.

In general, fresh martensite and tempered martensite have a high density of dislocations, so dislocations caused by plastic deformation are difficult to accumulate. Therefore, fresh martensite and tempered martensite have low work hardenability. By including fresh martensite and tempered martensite with low dislocation density in the metal structure, the work hardening ability of fresh martensite and tempered martensite can be increased, and as a result, the ductility of the hot stamped compact can be increased. I can do it.

In order to obtain the above-mentioned hot-stamped molded product, it is effective to perform sub-zero treatment, which involves hot-stamping under desired conditions and then cooling to a temperature range of −50° C. or lower.

Hereinafter, the hot stamp molded article according to this embodiment will be described in detail. First, the reason for limiting the chemical composition of the hot-stamped molded article according to this embodiment will be explained.

Note that the numerically limited ranges described below with "~" in between include the lower limit and the upper limit. Numerical values indicated as "less than" or "greater than" do not include the value within the numerical range. All percentages regarding chemical composition indicate mass %.

The hot-stamped molded article according to the present embodiment has a chemical composition in mass %: C: 0.08 to 0.70%, Si: 0.100 to 3.000%, Mn: 0.100 to 3.000. %, P: 0.1000% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.1000% or less, Al: 3.0000% or less, B: 0.0005 to 0. 0200%, and the remainder: contains Fe and impurities.
Each element will be explained below.

C: 0.08-0.70%
C is an element that improves the strength of the hot stamp molded product. If the C content is less than 0.08%, the desired strength cannot be obtained in the hot-stamped molded product. Therefore, the C content is set to 0.08% or more. The C content is preferably 0.10% or more, 0.15% or more, or 0.20% or more.
On the other hand, if the C content exceeds 0.70%, excellent ductility cannot be obtained in the hot stamped product. Therefore, the C content is set to 0.70% or less. The C content is preferably 0.65% or less, 0.60% or less, or 0.50% or less.

Si: 0.100-3.000%
Si is an element that improves the strength of the hot stamp molded product through solid solution strengthening. If the Si content is less than 0.100%, the desired strength cannot be obtained in the hot-stamped molded product. Therefore, the Si content is set to 0.100% or more. The Si content is preferably 0.300% or more, 0.500% or more, 0.800% or more, or 1.000% or more, more preferably more than 1.000% or 1.300% or more.
On the other hand, if the Si content exceeds 3.000%, excellent ductility cannot be obtained in the hot stamped product. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.800% or less, 2.500% or less, or 2.000% or less.

Mn: 0.100-3.000%
Mn is an element that improves the hardenability of steel. If the Mn content is less than 0.100%, the hardenability cannot be sufficiently improved, and the strength of the hot-stamped product decreases. Therefore, the Mn content is set to 0.100% or more. The Mn content is preferably 0.500% or more, 1.000% or more, 1.200% or 1.500% or more.
On the other hand, if the Mn content exceeds 3.000%, cracks due to Mn segregation are likely to occur, making it impossible to obtain excellent ductility in the hot stamped product. Therefore, the Mn content is set to 3.000% or less. Preferably, the Mn content is 2.700% or less, 2.500% or less, 2.300% or less or 2.000% or less.

P: 0.1000% or less P is an element that segregates at grain boundaries and becomes a starting point for fracture. If the P content exceeds 0.1000%, the occurrence of fracture becomes significant and the ductility of the hot stamped product deteriorates. Therefore, the P content is set to 0.1000% or less. The P content is preferably 0.0500% or less or less than 0.0020%.
The P content may be 0%, but if the P content is reduced to less than 0.0001%, the cost for removing P will increase significantly, which is economically unfavorable. Therefore, the P content may be set to 0.0001% or more.

S: 0.0100% or less S is an element that forms inclusions in steel. This inclusion becomes the starting point of destruction. When the S content exceeds 0.0100%, the occurrence of fracture becomes significant and the ductility of the hot-stamped molded product deteriorates. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less or 0.0050% or less.
The S content may be 0%, but if the S content is reduced to less than 0.0001%, the cost for removing S will increase significantly, which is economically unfavorable. Therefore, the S content may be set to 0.0001% or more.

N: 0.0200% or less N is an element that forms nitrides in steel. This nitride becomes a starting point for destruction. If the N content exceeds 0.0200%, the occurrence of destruction becomes significant. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0100% or less or 0.0050% or less.
The N content may be 0%, but if it is reduced to less than 0.0001%, the cost for removing N will increase significantly, which is economically unfavorable. Therefore, the N content may be set to 0.0001% or more.

O: 0.1000% or less O is an element that forms coarse oxides that become the starting point of fracture when contained in large amounts in steel. If the O content exceeds 0.1000%, the occurrence of fracture becomes significant and the ductility of the hot stamped product deteriorates. Therefore, the O content is set to 0.1000% or less. The O content is preferably 0.0800% or less, 0.0500% or less, 0.0100% or less, or less than 0.0020%.
The O content may be 0%, but may be set to 0.0005% or more or 0.0010% or more in order to disperse a large number of fine oxides during deoxidation of molten steel.

Al: 3.0000% or less Al is an element that has the effect of deoxidizing molten steel and making the steel sound (suppressing the occurrence of defects such as blowholes in the steel). However, when the Al content exceeds 3.0000%, coarse oxides are generated in the steel, resulting in deterioration of the ductility of the hot stamped body. Therefore, the Al content is set to 3.0000% or less. The Al content is preferably 2.5000% or less, 2.0000% or less, 1.5000% or less, or less than 0.0020%.
Although the Al content may be 0%, it may be 0.0001% or more.

B: 0.0005-0.0200%
B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, the desired strength cannot be obtained in the hot stamp molded product. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more or 0.0015% or more.
On the other hand, if the B content exceeds 0.0200%, the ductility of the hot-stamped body deteriorates. Therefore, the B content is set to 0.0200% or less. The B content is preferably 0.0150% or less and 0.0100% or less.

The remainder of the chemical composition of the hot-stamped molded body may be Fe and impurities. Examples of impurities include elements that are unavoidably mixed in from steel raw materials or scraps and/or during the steel manufacturing process and are allowed within a range that does not impede the properties of the hot-stamped molded product according to the present embodiment.

The chemical composition of the hot-stamped molded body may contain the following elements as optional elements in place of a part of Fe. When the following arbitrary elements are not included, the content is 0%.

Nb: 0.001-0.100%
Nb is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. To ensure this effect, the Nb content is preferably 0.001% or more.
On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides will be generated in the steel, resulting in deterioration of the ductility of the hot stamped body. Therefore, the Nb content is set to 0.100% or less.

Ti: 0.001-0.200%
Ti is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. To ensure this effect, the Ti content is preferably 0.001% or more.
On the other hand, if the Ti content exceeds 0.200%, a large amount of carbonitrides will be generated in the steel, resulting in deterioration of the ductility of the hot stamped body. Therefore, the Ti content is set to 0.200% or less.

Cr:0.01~1.00%
Cr is an element that increases the strength of the hot-stamped molded product by forming a solid solution in the prior austenite grains during heating before hot-stamping. To ensure this effect, the Cr content is preferably 0.01% or more.
On the other hand, if the Cr content is more than 1.00%, the ductility of the hot-stamped body will deteriorate. Therefore, the Cr content is set to 1.00% or less.

Mo: 0.01~1.00%
Mo is an element that increases the strength of the hot-stamped molded product by forming a solid solution in the prior austenite grains during heating before hot-stamping. To ensure this effect, the Mo content is preferably 0.01% or more.
On the other hand, if the Mo content exceeds 1.00%, the ductility of the hot-stamped compact will deteriorate. Therefore, the Mo content is set to 1.00% or less.

Co:0.01~5.00%
Co is an element that improves the strength of the hot stamp molded product through solid solution strengthening. To ensure this effect, the Co content is preferably 0.01% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Co content is set to 5.00% or less.

Ni: 0.01-3.00%
Ni is an element that increases the strength of the hot-stamped molded product by forming a solid solution in the prior austenite grains during heating before hot-stamping. To ensure this effect, the Ni content is preferably 0.01% or more.
On the other hand, since the above effect is saturated even if Ni is contained in a large amount, it is preferable that the Ni content is 3.00% or less.

Cu: 0.01~3.00%
Cu is an element that increases the strength of the hot-stamped molded product by solidly dissolving in the prior austenite grains during heating before hot-stamping. To ensure this effect, the Cu content is preferably 0.01% or more.
On the other hand, since the above effects are saturated even if Cu is contained in a large amount, the Cu content is preferably 3.00% or less.

V:0.01~3.00%
V is an element that forms carbonitrides in the steel and improves the strength of the hot stamped product through precipitation strengthening. To ensure this effect, the V content is preferably 0.01% or more.
On the other hand, if the V content exceeds 3.00%, a large amount of carbonitrides will be generated in the steel, resulting in deterioration of the ductility of the hot stamped body. Therefore, the V content is set to 3.00% or less.

W: 0.01~3.00%
W is an element that improves the strength of the hot stamp molded product. To ensure this effect, the W content is preferably 0.01% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the W content is set to 3.00% or less.

Ca: 0.0001-1.0000%
Ca is an element that suppresses the formation of oxides that become the starting point of destruction. To ensure this effect, the Ca content is preferably 0.0001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Ca content is set to 1.0000% or less.

Mg: 0.0001-1.0000%
Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, and has the effect of refining the metal structure. To ensure these effects, the Mg content is preferably 0.0001% or more.
On the other hand, if the Mg content exceeds 1.0000%, oxides in the steel increase and the ductility of the hot stamped product deteriorates. Therefore, the Mg content is set to 1.0000% or less.

REM: 0.0001-1.0000%
REM is an element that suppresses the formation of oxides that become the starting point of destruction. To ensure this effect, the REM content is preferably 0.0001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the REM content is set to 1.0000% or less.
In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements.

Sb: 0.01-1.00%
Sb is an element that improves the ductility of the hot-stamped compact by suppressing the formation of oxides that become a starting point for fracture. To ensure this effect, the Sb content is preferably 0.01% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Sb content is set to 1.00% or less.

Zr: 0.01~1.00%
Zr is an element that contributes to inclusion control, particularly to fine dispersion of inclusions, and increases the ductility of the hot-stamped compact. To ensure this effect, the Zr content is preferably 0.01% or more.
On the other hand, if a large amount of Zr is contained, the surface quality may deteriorate significantly. Therefore, the Zr content is set to 1.00% or less.

Sn: 0.01-1.00%
Sn is an element that suppresses the formation of oxides that become a starting point of fracture and contributes to improving the ductility of the hot-stamped compact. To ensure this effect, the Sn content is preferably 0.01% 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 1.00% or less.

As: 0.0001-1.0000%
As is an element that contributes to improving the ductility of the hot-stamped compact by reducing the austenite single-phase temperature, thereby making the prior austenite grains finer. To ensure this effect, the As content is preferably 0.0001% 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 1.0000% or less.

The chemical composition of the hot-stamped molded article described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured using a combustion-infrared absorption method, N using an inert gas melting-thermal conductivity method, and O using an inert gas melting-non-dispersive infrared absorption method.
When the hot-stamped molded body has a plating layer on its surface, the chemical composition may be analyzed after removing the plating layer by mechanical grinding.

Next, the metallographic structure of the hot-stamped molded body according to this embodiment will be explained.
The hot-stamped molded article according to the present embodiment has, in terms of area %, fresh martensite and tempered martensite: 60% or more in total, retained austenite: 2 to 30%, bainite: 38% or less, and Ferrite and pearlite: 2% or less in total, and when the number density of the retained austenite is ρ ₀ and the number density of the retained austenite after sub-zero treatment is ρ _S , ρ _S /ρ ₀ is 0.95 to 0. 1.01, the grain size is 5 to 10 μm, and the GAM value is 2° or more. The total area ratio of the fresh martensite and the tempered martensite is f ₀ , and the grain size is 5 to 10 μm. When the sum of the area ratios of the fresh martensite and the tempered martensite after the sub-zero treatment which are 10 μm and have a GAM value of 2° or more is _fS , _fS / _f0 is 0.80. ~1.20, and the half-width of the fresh martensite and the tempered martensite obtained by X-ray diffraction is β ₀ , and the fresh martensite after the sub-zero treatment and the tempered martensite obtained by X-ray diffraction are When the half width of tempered martensite is β _S , β _S /β ₀ is 0.990 to 1.010.

In this embodiment, the metal structure is defined at a position of 1/4 of the plate thickness from the surface (an area from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface). The reason is that the metal structure at this position shows a typical metal structure of a steel plate.
In addition, the surface here refers to the interface between the plating layer and the base steel plate when the hot-stamped molded body has a plating layer on the surface.

Furthermore, the subzero processing herein refers to subzero processing in tissue examination, and specifically refers to the following processing. Note that this is different from sub-zero processing in the manufacturing method described later.
In order to bring the temperature of the hot-stamped molded product to room temperature (20-28°C), it is held in the temperature range of 20-28°C for 60 minutes or more. Thereafter, it is cooled to a temperature range of -180 to -196°C at an average cooling rate of 10°C/s or more, and held in the temperature range for 20 to 40 minutes. After that, it is left in the air and the temperature is raised to room temperature.
As a method for cooling and maintaining the temperature within the above temperature range, for example, a method of immersing the hot-stamped molded body in liquid nitrogen can be mentioned.

Area ratio of fresh martensite and tempered martensite: 60% or more in total When the area ratio of fresh martensite and tempered martensite is less than 60% in total, it is difficult to obtain the desired strength in the hot-stamped molded product. Can not. Therefore, the total area ratio of fresh martensite and tempered martensite is 60% or more. Preferably it is more than 65%, 70% or more, 80% or more, or 90% or more.
Although the upper limit is not particularly specified, it may be set to 98% or less in view of the relationship with the area ratio of retained austenite.
Note that it is not necessary to contain both fresh martensite and tempered martensite, and it is also possible to contain only one of them, and the area ratio thereof may be 60% or more.

Area ratio of retained austenite: 2 to 30%
If the area ratio of retained austenite is less than 2%, desired ductility cannot be obtained in the hot stamped product. Therefore, the area ratio of retained austenite is set to 2% or more. The area ratio of retained austenite is preferably 5% or more, 10% or more, or 12% or more.
On the other hand, if the area ratio of retained austenite exceeds 30%, desired strength cannot be obtained in the hot-stamped molded product. Therefore, the area ratio of retained austenite is set to 30% or less. Preferably it is 25% or less, 20% or less, or 15% or less.

Area ratio of bainite: 38% or less If the area ratio of bainite exceeds 38%, desired strength cannot be obtained in the hot-stamped molded product. Therefore, the area ratio of bainite is set to 38% or less. The area ratio of bainite is preferably 30% or less, 20% or less, 15% or less, 10% or less, or 5% or less, and more preferably 0%.

Ferrite and pearlite: 2% or less in total If the total area ratio of ferrite and pearlite exceeds 2%, desired strength and ductility cannot be obtained in the hot-stamped molded product. Therefore, the total area ratio of ferrite and pearlite is 2% or less. The area ratio of ferrite and pearlite is preferably 1% or less, more preferably 0%.

The area ratio of each structure of the hot stamp molded body is obtained by the following method.
A sample is cut out from an arbitrary position 50 mm or more away from the end face of the hot-stamped molded body (if a sample cannot be taken from this position, a position avoiding the end) so that the plate thickness cross section can be observed. Although the size of the sample depends on the measuring device, it should be large enough to allow observation of about 10 mm in the direction perpendicular to the plate thickness direction.

After mirror-polishing the cross-section of the sample plate as an observation surface, in order to specify the photographic field of view, measure the area from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface using a micro Vickers hardness tester. Make impressions at 10 locations using an indentation load of 100 gf. Next, the observation surface is immersed in an acetylacetone-based electrolytic solution to perform electrolytic etching. As a result, the morphology of iron-based carbides contained in the structure becomes clearer, and the contrast of grain boundaries becomes clearer.

Next, using a field emission scanning electron microscope (FE-SEM) equipped with a secondary electron detector, the field of view of the 10 locations where the indentation was made was imaged at a magnification of 5000x. Take a secondary electron image. In the obtained photographs, ferrite, pearlite, and structures other than ferrite and pearlite are distinguished. The area ratio of ferrite is obtained by calculating the average value of the area ratio of regions determined to be ferrite. Similarly, the area ratio of pearlite is obtained by calculating the average value of the area ratio of regions determined to be pearlite.
Note that a structure that is a massive crystal grain and does not include a substructure such as a lath inside the structure is regarded as ferrite. A structure in which plate-shaped ferrite and Fe-based carbide are layered is considered to be pearlite.

A secondary electron image is photographed at a magnification of 10,000 times for the same photographic field of view. In the obtained photographs, each tissue is identified as follows.
A structure that is an aggregation of lath-shaped crystal grains and contains Fe-based carbides with a major axis of 20 nm or more and extending in different directions inside the structure is considered to be tempered martensite.
Among the structures that are a collection of lath-shaped crystal grains and do not contain Fe-based carbides with a major axis of 20 nm or more inside the structure, there are structures in which Fe-based carbides are precipitated between the laths, and structures in which Fe-based carbides are precipitated inside the laths. A structure in which Fe-based carbides extend in the same direction is regarded as bainite.
Here, Fe-based carbide extending in the same direction refers to one in which the difference in the elongation direction of the Fe-based carbide is within 5°.
By calculating the area ratio of the region determined to be tempered martensite or bainite, the area ratio of tempered martensite and the area ratio of bainite are obtained.

The area ratio of retained austenite is measured in the same photographic field of view as when the photographed photograph was taken. After polishing the observation surface of the above sample using #600 to #1500 silicon carbide paper, polish it to a mirror surface using a liquid made by dispersing diamond powder with a particle size of 1 to 6 μm in diluted liquid such as alcohol or pure water. Finish. Next, the sample is polished for 8 minutes using colloidal silica without an alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. Crystal orientation information is obtained by measuring by electron backscatter diffraction at a measurement interval of 0.4 μm in the same photographic field of view as when the photograph was taken. For the measurement, an EBSD analysis 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 degree of vacuum in the EBSD analyzer was 9.6 × 10 ^-5 Pa or less, the acceleration voltage was 25 kV, the irradiation current level was 14, the electron beam irradiation level was 62, the operating distance was 15 mm, and the sample inclination angle was The angle shall be 70°.

From the obtained crystal orientation information, using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, a region whose crystal structure is fcc is determined to be retained austenite. By calculating the area ratio of this retained austenite region, the area ratio of retained austenite is obtained.
The area ratio of fresh martensite is obtained by subtracting the area ratio of ferrite, pearlite, tempered martensite, bainite, and retained austenite obtained by the above method from 100%.

ρ _S /ρ ₀ : 0.95 to 1.01
In the hot-stamped molded article according to this embodiment, the amount of retained austenite, which has low stability, is reduced. Therefore, even if the subzero treatment in the above-mentioned microstructure inspection is performed, the amount of retained austenite hardly changes.

When the number density of retained austenite is ρ ₀ and the number density of retained austenite after sub-zero treatment in the above-mentioned microstructure inspection is ρ _S , ρ _S /ρ ₀ is less than 0.95, which indicates low stability. This indicates that the amount of retained austenite decreased because the amount of retained austenite was large. If the amount of residual austenite with low stability is large, desired ductility cannot be obtained in the hot stamped product. Therefore, ρ _S /ρ ₀ is set to 0.95 or more. ρ _S /ρ ₀ is preferably 0.96 or more, 0.97 or more, or 0.98 or more.
Generally, the amount of retained austenite does not increase due to sub-zero treatment, so ρ _S /ρ ₀ becomes 1.01 or less.

ρ _S and ρ ₀ are obtained by the following method.
The retained austenite in the observation area is identified for the sample taken from the hot-stamped molded body using the same method as when measuring the area ratio of each structure described above. The number of retained austenites in the observation area is determined and the number is divided by the area of the observation area to obtain the number density ρ ₀ of retained austenite.
The number density ρ _S of retained austenite after the sub-zero treatment is obtained by measuring the hot-stamped molded body after the sub-zero treatment in the above-described microstructure inspection using a similar method.

_fS / _f0 : 0.80 to 1.20
Generally, when a hot-stamped molded body is subjected to sub-zero treatment, retained austenite transforms into fresh martensite and tempered martensite, thereby increasing the amount of fresh martensite and tempered martensite.

The sum of the area ratios of fresh martensite and tempered martensite with a grain size of 5 to 10 μm and a GAM value of 2° or more is f ₀ , and a grain size of 5 to 10 μm and a GAM value is 2° or more. When the sum of the area ratios of fresh martensite and tempered martensite after sub-zero treatment in the above-mentioned microstructure inspection is f _S , f _S /f ₀ is more than 1.20. This shows that the amount of fresh martensite and tempered martensite transformed from retained austenite is large. The retained austenite transformed into fresh martensite and tempered martensite by the above-mentioned sub-zero treatment is retained austenite with low stability. In other words, f _S /f ₀ exceeding 1.20 indicates that the amount of retained austenite with low stability was large. If the amount of residual austenite with low stability is large, desired ductility cannot be obtained in the hot stamped product. Therefore, f _S /f ₀ is set to 1.20 or less. f _S /f ₀ is preferably 1.15 or less, 1.10 or less, 1.05 or less, or 1.01 or less.

On the other hand, f _S /f ₀ of less than 0.80 indicates that the dislocations in fresh martensite and tempered martensite were annihilated by compression due to thermal contraction due to the above-described sub-zero treatment. In other words, the amount of fresh martensite and tempered martensite with high dislocation density was large. Fresh martensite and tempered martensite, which have a high dislocation density, have low work hardening ability, so if their amounts are large, it is impossible to obtain the desired ductility in the hot-stamped compact. Therefore, f _S /f ₀ is set to 0.80 or more. f _S /f ₀ is preferably 0.85 or more, 0.90 or more, 0.95 or more, 0.97 or more, 0.98 or more, or 0.99 or more.

Here, fresh martensite and tempered martensite with a grain size of 5 to 10 μm and a GAM value of 2° or more are stipulated if the grain size is less than 5 μm or the GAM value is less than 2°. This is because fresh martensite and tempered martensite may not be accurately measured by EBSD analysis. In addition, fresh martensite and tempered martensite with a grain size of more than 10 μm have a low dislocation density and are unlikely to undergo structural changes due to sub-zero treatment, so they do not affect the ductility of the hot stamped compact. It is.

f _S and f ₀ are obtained by the following method.
Fresh martensite and tempered martensite in the observation region are identified for the sample taken from the hot-stamped molded body using the same method as when measuring the area ratio of each structure described above. For each fresh martensite and each tempered martensite in the observation area, the grain size is calculated using the "Grain Size" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. From these, fresh martensite and tempered martensite with a particle size of 5 to 10 μm are specified.

Next, for fresh martensite and tempered martensite with a grain size of 5 to 10 μm in the observation area, we used the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" that comes with the EBSD analysis device. A region in which the orientation difference (GAM value: Grain Average Misorientation) within a crystal grain is 2° or more is specified using the method. By calculating the area ratio of the specified region, the total area ratio f _S of fresh martensite and tempered martensite having a grain size of 5 to 10 μm and a GAM value of 2° or more is obtained.
By measuring the hot-stamped molded product after the sub-zero treatment in the above-mentioned microstructure inspection using the same method, it was found that the fresh marten after the sub-zero treatment has a grain size of 5 to 10 μm and a GAM value of 2° or more. The sum of the area fractions of site and tempered martensite f _S is obtained.

β _S /β ₀ : 0.990 to 1.010
The half-width of fresh martensite and tempered martensite obtained by X-ray diffraction is β ₀ , and the half-width of fresh martensite and tempered martensite obtained by X-ray diffraction after sub-zero treatment in the above-mentioned structure inspection is When β _S is defined as β S , the fact that β _S /β ₀ is more than 1.010 indicates that the amount of fresh martensite and tempered martensite transformed from retained austenite is large. The retained austenite transformed into fresh martensite and tempered martensite by the above-mentioned sub-zero treatment is retained austenite with low stability. In other words, β _S /β ₀ exceeding 1.010 indicates that the amount of retained austenite with low stability was large. If the amount of residual austenite with low stability is large, desired ductility cannot be obtained in the hot stamped product. Therefore, β _S /β ₀ is set to 1.010 or less. β _S /β ₀ is preferably 1.005 or less or 1.001 or less.

On the other hand, the fact that β _S /β ₀ is less than 0.990 indicates that the dislocations in fresh martensite and tempered martensite were annihilated by compression due to thermal contraction due to the sub-zero treatment in the above-mentioned structure inspection. In other words, the amount of fresh martensite and tempered martensite with high dislocation density was large. Fresh martensite and tempered martensite, which have a high dislocation density, have low work hardening ability, so if their amounts are large, it is impossible to obtain the desired ductility in the hot-stamped compact. Therefore, β _S /β ₀ is set to 0.990 or more. β _S /β ₀ is preferably 0.995 or more, 0.997 or more, or 0.998 or more.

β _S and β ₀ are obtained by the following method.
Observe a cross section perpendicular to the plate thickness direction (a cross section parallel to the plate surface) from any position 50 mm or more away from the edge of the hot stamped product (if a sample cannot be collected from this position, avoid the edge) Cut out a square sample about 10 mm square. Next, by chemical polishing, the thickness of the sample was reduced so that a cross section parallel to the plate surface could be observed at any position from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface. Reduce the thickness. Chemical polishing may be performed, for example, by immersing the sample in an aqueous solution of phosphoric acid and hydrogen peroxide at room temperature.

After chemical polishing, an X-ray diffraction profile is obtained by performing X-ray diffraction on the cross section parallel to the plate surface. In the obtained X-ray diffraction profile, the three peaks of bcc-Fe (110), (200), and (211) are fitted with the Voigt function, and fresh martensite and tempered martensite are determined. The half width β _{0 (hkl)} (β _{0 (110)} , β _{0 (200)} and β _{0 (211)} ) are obtained.
By measuring the hot stamped compact after the sub-zero treatment in the above-mentioned microstructure inspection using the same method, the half-width β _{S (hkl)} (β _{S (110 )} , β _{S (200)} and β _{S (211)} ) are obtained.
The value of the difference in half width of each diffraction surface |β _{S (hkl)} −β _{0 (hkl)} | that is closest to 1.000 is defined as β _s /β ₀ .
For the measurement, it is preferable to use an X-ray diffraction device (XRD: RINT2500 manufactured by Rigaku Corporation).

Tensile strength: 700 MPa or more The hot stamp molded article according to the present embodiment may have a tensile strength of 700 MPa or more. If the tensile strength is 700 MPa or more, it can contribute to reducing the weight of the vehicle body.
Although the upper limit is not particularly specified, it may be 4000 MPa or less.

Uniform elongation: 3.0% or more The hot-stamped molded article according to the present embodiment may have a uniform elongation, which is an index of ductility, of 3.0% or more. The upper limit is not particularly limited, but may be 40.0% or less.

The tensile strength and uniform elongation are measured by performing a tensile test in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The tensile test piece is taken at the center of the plate surface of the hot-stamped body (avoiding the edges), and the test piece is taken with the direction parallel to the longitudinal direction of the hot-stamped body considered as the longitudinal direction.
Note that uniform elongation refers to "full elongation at maximum test force" as defined in JIS Z 2241:2011.

The hot stamp molded article according to this embodiment may have a plating layer on the surface. By having a plating layer on the surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanizing layer, an electrogalvanizing layer, an alloyed hot-dip galvanizing layer, and the like.

Next, a hot stamping steel plate for obtaining a hot stamping molded article according to the present embodiment will be explained.
The hot stamping steel plate has the above-mentioned chemical composition. The metal structure of the steel sheet for hot stamping is not particularly limited as long as desired strength and ductility can be obtained after hot stamping, but for example, in terms of area ratio, ferrite: 0 to 90%, bainite and martensite: 0 to 100%, It may consist of pearlite: 0 to 80% and retained austenite: 0 to 5%.

Moreover, the steel plate for hot stamping may have a plating layer on the surface. By having a plating layer on the surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanizing layer, an electrogalvanizing layer, an alloyed hot-dip galvanizing layer, and the like.

Method for manufacturing a steel plate for hot stamping Hereinafter, the method for manufacturing a steel plate for hot stamping to obtain a hot stamping molded body according to the present embodiment is not particularly limited, and may be manufactured under normal conditions.

A hot-stamped molded body according to this embodiment is obtained by hot-stamping a hot-stamping steel plate and then performing subzero treatment. Note that the subzero treatment in the manufacturing method is different from the subzero treatment in the tissue inspection described above.
Note that all temperatures mentioned below refer to the surface temperature of the steel plate.

Hot Stamping Conditions for hot stamping are not particularly limited, but for example, it is preferable to heat a steel plate for hot stamping to a temperature range of 800 to 1000°C, hold it in this temperature range for 60 to 1200 seconds, and then hot stamp. After hot stamping, it is preferable to adopt, for example, any one of the following conditions (I) to (IV).
(I) Hot stamp and perform subzero processing as is.
(II) After hot stamping and rapidly cooling to a desired temperature range, in order to increase the amount of retained austenite, perform sub-zero treatment after cooling to a temperature range of 50°C or less at an average cooling rate of 30°C/s or less. .
(III) After hot stamping and rapidly cooling to a desired temperature range, intermediate holding is performed in the temperature range, and sub-zero treatment is performed.
(IV) After hot-stamping and rapidly cooling to a desired temperature range, and performing intermediate holding in the temperature range, reheating is performed, and then sub-zero treatment is performed.

(I) When performing subzero processing directly after hot stamping, it is preferable to use the following method.
After hot stamping, it is cooled to a temperature range of -50 to -196°C at an average cooling rate of 5°C/s or more. After maintaining the temperature range for 1 minute or more, leave it in the atmosphere. In order to uniformly cool the inside of the hot-stamped molded product, it is sufficient to hold it in the above temperature range for 5 minutes or more. In the sub-zero treatment, the average cooling rate to the above temperature range is more preferably 10°C/s or more, 30°C/s or more, or 50°C/s or more.
By performing the subzero treatment under the above conditions, fresh martensite and tempered martensite with low dislocation density can be included in the metal structure, and β _S /β ₀ can be controlled within a preferable range.

Examples of methods for cooling and holding under the above conditions include methods using dry ice, carbon dioxide, or liquid nitrogen as a solvent. In these methods, in order to further increase the cooling rate or to uniformly cool the inside of the hot-stamped body, the solvent may be stirred or the hot-stamped body may be caused to migrate in the solvent.
Other methods for cooling and maintaining under the above conditions include, for example, a method using a cryogenic freezer. In this method, in order to further increase the cooling rate or uniformly cool the inside of the hot-stamped molded product, the air within the refrigerator may be circulated or the hot-stamped molded product may be caused to migrate within the refrigerator.

(II) When rapidly cooling to a desired temperature range by hot stamping and then cooling to a temperature range of 50°C or less at an average cooling rate of 30°C/s or less, the following method is preferable.
After cooling by hot stamping at an average cooling rate of Vc90 (°C/s) or more to a cooling stop temperature of Mf (°C) or more and less than 500°C, the temperature range is 30°C/s or less to a temperature range of 50°C or less. Cool at an average cooling rate of At this time, cooling to a temperature range of 50° C. or lower may be performed to about room temperature (20 to 28° C.). After that, subzero processing is performed.
By setting the average cooling rate to a cooling stop temperature of Mf (°C) or higher and lower than 500°C to a critical cooling rate of Vc90 (°C/s) or higher, the generation of ferrite and pearlite can be suppressed. In addition, by setting the cooling stop temperature of cooling in which the average cooling rate is equal to or higher than the critical cooling rate Vc90 (°C/s) to a temperature range of Mf (°C) or higher and lower than 500°C, the generation of bainite can be suppressed. .

The subzero processing in this case is preferably performed using the following method. After hot stamping, it is cooled to a temperature range of -50 to -196°C at an average cooling rate of 5°C/s or more. After maintaining the temperature range for 1 minute or more, leave it in the atmosphere. In the sub-zero treatment, the average cooling rate to the above temperature range is more preferably 10°C/s or more, 30°C/s or more, or 50°C/s or more.
By performing subzero treatment under the above conditions after cooling, the amount of residual austenite with low stability can be reduced, and fresh martensite and tempered martensite with low dislocation density can be included in the metal structure. As a result, ρ _S /ρ ₀ , f _S /f ₀ and β _S /β ₀ can be controlled within desired ranges.

Vc90 (°C/s) can be expressed by the following formula (1).
Vc90 (℃/s)=10 ^{2.94-0.75×{(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr+2×Mo)-1}…} (1)
The element symbol in the above formula (1) indicates the content in mass % of each element, and when the element is not contained, 0 is substituted.

(III) When performing intermediate holding after hot stamping, it is preferable to use the following method.
After hot stamping, it is cooled to a temperature range of Ms (°C) to Mf (°C) at an average cooling rate of at least a critical cooling rate of Vc90 (°C/s). It is held in the temperature range for 5 seconds or more and less than 1200 seconds, and then cooled to a temperature range of 50°C or less at an average cooling rate of 0.5°C/s or more. At this time, cooling to a temperature range of 50° C. or lower may be performed to about room temperature (20 to 28° C.). After that, subzero processing is performed.

The subzero processing here is preferably performed under the same conditions as the subzero processing of condition (II) described above.
By performing sub-zero treatment under the above conditions after intermediate holding, it is possible to reduce the amount of residual austenite with low stability, and it is possible to include fresh martensite and tempered martensite with low dislocation density in the metal structure. . As a result, ρ _S /ρ ₀ , f _S /f ₀ and β _S /β ₀ can be controlled within desired ranges.

By setting the holding time in the temperature range of Ms (°C) to Mf (°C) to less than 1200 seconds, a desired amount of retained austenite can be obtained. Furthermore, by setting the average cooling rate in the temperature range of Ms (°C) to Mf (°C) to a critical cooling rate of Vc90 (°C/s) or more, it is possible to suppress the formation of ferrite and pearlite. In addition, by setting the cooling stop temperature of cooling where the average cooling rate is equal to or higher than the critical cooling rate Vc90 (°C/s) to a temperature range of Ms (°C) to Mf (°C), excessive generation of bainite is suppressed, A desired amount of fresh martensite and tempered martensite can be obtained.

When maintaining the temperature in the temperature range of Ms (°C) to Mf (°C), the temperature may be kept constant or may be varied within this temperature range.

(IV) When carrying out intermediate holding and reheating after hot stamping, it is preferable to use the following method.
After hot stamping, it is cooled to a temperature range of Ms (°C) to Mf (°C) at an average cooling rate of at least a critical cooling rate of Vc90 (°C/s), and held in the temperature range for 5 seconds or more and less than 1200 seconds. Next, after reheating to a temperature range of Mf (°C) or more and less than 600°C, and holding in the temperature range for 10 seconds or more but less than 1200 seconds, 50°C at an average cooling rate of 0.5°C/s or more. Cool to a temperature range below ℃. At this time, cooling to a temperature range of 50° C. or lower may be performed to about room temperature (20 to 28° C.). After that, subzero processing is performed.

The subzero processing here is preferably performed under the same conditions as the subzero processing of condition (II) described above.
By performing sub-zero treatment under the above conditions after intermediate holding and reheating, the amount of residual austenite with low stability can be reduced, and the metal structure can include fresh martensite and tempered martensite with low dislocation density. be able to. As a result, ρ _S /ρ ₀ , f _S /f ₀ and β _S /β ₀ can be controlled within desired ranges.

By setting the average cooling rate in the temperature range of Ms (°C) to Mf (°C) to a critical cooling rate of Vc90 (°C/s) or higher, the formation of bainite, ferrite, and pearlite can be suppressed. In addition, by setting the cooling stop temperature of cooling where the average cooling rate is equal to or higher than the critical cooling rate Vc90 (°C/s) to a temperature range of Ms (°C) to Mf (°C), excessive generation of bainite is suppressed, A desired amount of fresh martensite and tempered martensite can be obtained. Further, by setting the holding time in the temperature range of Ms (°C) to Mf (°C) during intermediate holding to less than 1200 seconds, a desired amount of retained austenite can be obtained. In addition, by reheating to a higher temperature than the temperature during intermediate holding and setting the holding temperature during reheating to a temperature range of Mf (°C) or more and less than 600°C, a desired amount of retained austenite can be obtained. . Further, by setting the holding time during reheating to 1200 seconds or less, a desired amount of retained austenite can be obtained.

Note that Ms (°C) can be represented by the following formula (2), and Mf (°C) can be represented by the following formula (3).
Ms(℃)=539-423×C-30×Mn-12×Cr-17×Ni-7.5×Mo…(2)
Mf (℃)=Ms-209...(3)
The element symbol in the above formula (2) indicates the content in mass % of each element, and when the element is not contained, 0 is substituted.

Examples of cooling in which the average cooling rate is equal to or higher than the critical cooling rate Vc90 (° C./s) include mold cooling, gas cooling, and water cooling.
Furthermore, in the present embodiment, the average cooling rate refers to a value obtained by dividing the range of temperature drop of the steel plate from the start of cooling to the time of completion of cooling by the time required from the start of cooling to the time of completion of cooling.

The hot-stamped molded article according to the present embodiment can be stably manufactured by the manufacturing method described above.

Next, an example of the present invention will be described. The conditions in the example are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is based on this example of conditions. It is not limited. 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 plate for hot stamping was manufactured under normal conditions using a steel piece manufactured by casting molten steel having the chemical composition shown in Table 1. In addition, the content described using "<" in the table indicates that the content was less than that value and more than 0. For example, the content described as "<0.0020" in the table indicates that the content was more than 0% and less than 0.0020%.

The obtained steel plate for hot stamping is heated to a temperature range of 800 to 1000°C, held in this temperature range for 60 to 1200 seconds, and then hot stamped, and then subjected to the above-mentioned conditions (I) to (IV) or Under different conditions, hot-stamped molded bodies shown in Table 3A and Table 3B were obtained.
In addition, production No. In No. 42, subzero processing was performed before hot stamping, and subzero processing was not performed after hot stamping. Manufacturing No. Sub-zero treatment at 42 is performed by cooling a steel plate for hot stamping to a temperature range of -50 to -196°C at an average cooling rate of 5°C/s or more, holding it in the temperature range for at least 1 minute, and then exposing it to air. The condition was that it be left inside.

Note that the subzero treatment under conditions (I) to (IV), the cooling under condition (II), the intermediate holding under condition (III), and the intermediate holding and reheating under condition (IV) are performed under the conditions shown in Table 2A and Table 2B. I went there.
In the example of cooling under condition (II), after cooling to the cooling stop temperature listed in the table, cooling to a temperature range of 50 ° C or less at the average cooling rate listed in the table, and then performing sub-zero treatment. Ta. Note that cooling to a temperature range of 50° C. or lower was performed to room temperature (20 to 28° C.).
In the example in which intermediate holding was performed under condition (III), after intermediate holding, the material was cooled to a temperature range of 50° C. or less at an average cooling rate of 0.5° C./s or more, and then subzero treatment was performed. Note that cooling to a temperature range of 50° C. or lower was performed to room temperature (20 to 28° C.).
In the example of condition (IV) in which intermediate holding and reheating were performed, after being held at the holding temperature for reheating, it was cooled to a temperature range of 50°C or less at an average cooling rate of 0.5°C/s or more. From this, subzero processing was performed. Note that cooling to a temperature range of 50° C. or lower was performed to room temperature (20 to 28° C.).

Note that the underline in the table indicates that it is outside the scope of the present invention, that it falls outside the preferred manufacturing conditions, or that the characteristic value is unfavorable.
The metallographic structure of the hot-stamped molded product was measured by the above-mentioned measurement method. In addition, the mechanical properties of the hot-stamped molded product were measured by the method described above.

If the tensile strength was 700 MPa or more, it was determined that the hot stamp molded product had high strength and passed. On the other hand, when the tensile strength was less than 700 MPa, it was determined that the hot stamp molded product did not have high strength and was rejected.

When the uniform elongation was 3.0% or more, it was determined that the hot stamped molded product had excellent ductility and passed. On the other hand, when the uniform elongation was less than 3.0%, it was determined that the hot stamp molded product did not have excellent ductility and was rejected.

In addition, the product of tensile strength and uniform elongation (tensile strength x uniform elongation) of the hot-stamped molded bodies according to the present invention examples and the hot-stamped molded bodies according to comparative examples was evaluated.

Looking at Tables 3A and 3B, it can be seen that the hot-stamped molded articles that are examples of the present invention have higher strength and superior ductility than the hot-stamped molded articles that are comparative examples.

Manufacturing No. Production No. 2 is an example of the present invention. Tensile strength×uniform elongation (MPa·%) was inferior compared to No. 1.
Manufacturing No. Production No. 4 is an example of the present invention. Compared to No. 3, the tensile strength x uniform elongation (MPa.%) was inferior.

Manufacturing No. Production No. 6 is an example of the present invention. Compared to No. 5, tensile strength x uniform elongation (MPa.%) was inferior.
Also, production No. 8 is production No. 8 in which cooling under condition (II) was not performed. Compared to No. 7, the amount of retained austenite was large and the uniform elongation was improved.

Manufacturing No. Production No. 9 is an example of the present invention. Compared to No. 8, tensile strength x uniform elongation (MPa·%) was inferior. In addition, the production No. 1 was subjected to sub-zero treatment before hot stamping, but was not subjected to sub-zero treatment after hot stamping. In No. 42, the structure was austenitized by heating during hot stamping, resulting in production No. It has the same structure as Manufacturing No. 9, which is an example of the present invention. Compared to No. 8, tensile strength x uniform elongation (MPa·%) was inferior.
Manufacturing No. Production No. 11 is an example of the present invention. Compared to No. 10, the tensile strength x uniform elongation (MPa·%) was inferior. Moreover, the tensile strength did not meet the acceptance criteria.
Manufacturing No. Production No. 13 is an example of the present invention. Compared to No. 12, the tensile strength x uniform elongation (MPa·%) was inferior.

Manufacturing No. 15 to 17 are production Nos. 15 to 17, which are examples of the present invention. Compared to No. 14, the tensile strength x uniform elongation (MPa·%) was inferior. Also, production No. In samples No. 16 and 17, the tensile strength did not meet the acceptance criteria.

Manufacturing No. Production No. 19 is an example of the present invention. Compared to No. 18, the tensile strength x uniform elongation (MPa·%) was inferior.
Manufacturing No. 21 and 23 are production Nos. 21 and 23, which are examples of the present invention. Compared to No. 20, the tensile strength x uniform elongation (MPa·%) was inferior. Also, production No. In samples No. 21 and No. 22, the tensile strength did not meet the acceptance criteria.

Manufacturing No. 25 is production No. 25, which is an example of the present invention. Compared to No. 24, the tensile strength x uniform elongation (MPa·%) was inferior.
Manufacturing No. Production No. 27 is an example of the present invention. Compared to No. 26, the tensile strength x uniform elongation (MPa·%) was inferior.

Manufacturing No. Production No. 29 is an example of the present invention. Compared to No. 28, the tensile strength x uniform elongation (MPa·%) was inferior.
Manufacturing No. Samples 30 to 32, 34, and 35 did not meet the acceptance criteria for tensile strength or uniform elongation. Also, production No. Production No. 33 is an example of the present invention having a similar chemical composition and production conditions. Compared to No. 18, the tensile strength x uniform elongation (MPa·%) was inferior.

Manufacturing No. 37 is production No. 37, which is an example of the present invention. Compared to No. 36, the tensile strength x uniform elongation (MPa·%) was inferior.
Manufacturing No. Production No. 39 is an example of the present invention. Compared to No. 38, the tensile strength x uniform elongation (MPa·%) was inferior.
Manufacturing No. 41 is production No. 41, which is an example of the present invention. Compared to No. 40, the tensile strength x uniform elongation (MPa·%) was inferior.

Claims

The chemical composition is in mass%,
C: 0.08-0.70%,
Si: 0.100-3.000%,
Mn: 0.100-3.000%,
P: 0.1000% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.1000% or less,
Al: 3.0000% or less,
B: 0.0005-0.0200%,
Nb: 0 to 0.100%,
Ti: 0-0.200%,
Cr: 0-1.00%,
Mo: 0-1.00%,
Co: 0-5.00%,
Ni: 0-3.00%,
Cu: 0-3.00%,
V: 0 to 3.00%,
W: 0-3.00%,
Ca: 0-1.0000%,
Mg: 0 to 1.0000%,
REM: 0-1.0000%,
Sb: 0 to 1.00%,
Zr: 0 to 1.00%,
Contains Sn: 0 to 1.00% and As: 0 to 1.0000%,
The remainder consists of Fe and impurities,
In metallographic structure,
In area%,
Fresh martensite and tempered martensite: 60% or more in total,
Retained austenite: 2-30%,
Bainite: 38% or less, and
Ferrite and pearlite: 2% or less in total,
When the number density of the retained austenite is ρ 0 , and the number density of the retained austenite after sub-zero treatment is ρ S , ρ S /ρ 0 is 0.95 to 1.01,
The total area ratio of the fresh martensite and the tempered martensite having a grain size of 5 to 10 μm and a GAM value of 2° or more is f0 , and the grain size is 5 to 10 μm, and When the sum of the area ratios of the fresh martensite and the tempered martensite after the sub-zero treatment with a GAM value of 2° or more is f S , f S /f 0 is 0.80 to 1.20. ,
The half-width of the fresh martensite and the tempered martensite obtained by X-ray diffraction is β 0 , and the half-width of the fresh martensite and the tempered martensite after the sub-zero treatment obtained by X-ray diffraction is β0. A hot stamp molded article characterized in that β S /β 0 is 0.990 to 1.010 .
The chemical composition is in mass%,
Nb: 0.001 to 0.100%,
Ti: 0.001 to 0.200%,
Cr: 0.01-1.00%,
Mo: 0.01-1.00%,
Co: 0.01-5.00%,
Ni: 0.01 to 3.00%,
Cu: 0.01-3.00%,
V: 0.01 to 3.00%,
W: 0.01-3.00%,
Ca: 0.0001-1.0000%,
Mg: 0.0001 to 1.0000%,
REM: 0.0001-1.0000%,
Sb: 0.01 to 1.00%,
Zr: 0.01-1.00%,
Sn: 0.01 to 1.00%, and As: 0.0001 to 1.0000%
The hot-stamped molded article according to claim 1, characterized in that it contains one or more selected from the group consisting of: